Immunology Today, voL 7, No. 11, 1986
TOROI
.TCHBOOK
In July over 700 delegates gathered in Toronto for the 6th International Congress of Immunology. In 5 days they were given an opportunity of attending 6 plenary lectures and, in the parallel sessions, 110 symposium talks +and 1100 workshop presentations. To review thes papers comprehensiw _
~ould be impractical and of dubious value (readers can expect, in any case, that important topics will continue to be reviewed at adequate length in issues of this journal). What follows is a collection of invited commentaries on some aspects of immunology in 1986, as reported at the Congress and certain satellite meetings. The correspondents were asked to select from the papers they heard what they thought was stimulating. They were brief, at the Editor's insistence, and so inevitably had to omit from their reports some r4ork worthy of attention. ~ _ _
This report attempts to highlight the new findings presented at the Congress on T-cell recognition of antigen, concentrating on the T-cell receptor, antigen processing and presentation, and the types of antigenic structures recognized by T cells. We do not attempt to cover other related topics such as the ontogeny of T cells or their receptors, or the regulatory function of T cells. The major histocompatibility complex (MHC)-restricted receptor on antigen-specific T cells is a disulfidelinked heterodimer composed of two subunits (e and 13)of 40-50 kDa each. There has been remarkable progress in the molecular analysis of the c~ and 13 genes encoding the T-cell receptor (TCR) over the past few years. In addition to the e and 13 genes, a third rearranged TCR gene, T~, has been identified. The identification of a protein molecule which could be the product of the T-/ gene was reported by M. Brenner (Boston). A population of T cells was found to express surface T3 but neither surface TCR c~nor 13. It lacked mRNA encoding TCR ct and 13chains but expressed T~/ mRNA. Chemical crosslinking and immunoprecipitation with anti-T3 identified two new nondisulfide-linked proteins of 55 kDa and 40 kDa. In addition, antisera raised against synthetic peptides rep~) 1986, Elsevier Science Publishers B.V., Amsterdam
Antigenrecognitionby T cells Alison Finnegan and JayA. Berzofsky2 resenting 17 amino acids in the V~/ gene and a 20 amino acid stretch in the C~ region immunoprecipitated the 55 kDa molecules from the T3+TCR - cells. Thus the 55 kDa molecule may be the protein encoded by the T~ gene. The additional molecule of 40 kDa is designated Ta. These two polypeptides may form a T3-associated, nondisulfide-linked heterodimer and a second TCR. Parallel results were reported by L. Chess (New York) examining a cloned T3+T4-T8 - human thymocyte cell line. This T-cell clone lacks expression of mature mRNA for c~ and 13 chains but expresses T-y mRNA, and precipitation with antiT3 identified a nondisulfide-linked complex that includes, in addition to T3, two new proteins of 44 kDa and 62 kDa. Antisera to a synthetic peptide corresponding to a portion of the constant region of the T~/ gene precipitated the 44 kDa molecule. A similar structure that could represent the -y gene product was reported by S. Tonegawa (Cambridge, M A ) i n L3T4- Lyt 2 thymocytes which express ~/mRNA but lack TCR ~ and 13 mRNA. Also, T. Mak (Toronto) re-
0167 - 4919186/$02.00
ported ~/mRNA expression in nylonpassed cells of nude mice which express neither c~ nor 13 transcripts. Thus, the ~I chain seems finally to have been found, but on cell populations which do not express the c~13 TCR. The function of these cell populations and of the receptor they express is not yet known. The relative contributions of the and 13 chains of the TCR in the recognition of MHC and antigen is poorly understood. F. Rupp (Zurich) demonstrated that two T-cell clones of different antigen and MHC specificities share the same idiotype and express the same V~ and V13. The two clones differ in the D-J(13) portion of the TCR 13 chain, so the D-J region may account for the differences in specificity of these T-cell clones. In addition, A. Blackman (Denver) demonstrated that determinants formed by the combination of and 13 chains contribute to the specificity of both chains. Interestingly, J. Kappler (Denver) reported an influence of H-2 haplotype on the usage of a particular V13 family (V138) which may reflect the effect of H-2 on T-cell specificities in the reper- 317
0
Immunology Today, vol. 7, No. 1 I, 1986
toire. Cell surface structures other than the TCR may be important for some T-cell clone specificity. The (~ and 13 genes from a cytotoxic T-cell (CTL) clone were transfected into another CTL clone as reported by S. Tonegawa but expression of these TCR chains was not sufficient to endow the recipient clone with the specific cytotoxicity of the donor clone. The ability to express and isolate V(~ and VI3 in soluble form attached to Ig constant regions, as described by M. Davis (Stanford), may ultimately allow the resolution of these questions by direct binding studies.
0
P.
318
Antigen-la interactions T helper/inducer cells usually recognize exogenous antigen in association with MHC class II molecules. In the mouse, cloned T cells selectively recognize different antigens in association with either the I-A or I-E class II molecules. The determinant selection hypothesis states that there is specificity to the interaction between the antigen and la molecules in the formation of a complex recognized by the T cells. E. Unanue (St. Louis) and H. Grey (Denver) showed by equilibrium dialysis that certain peptides bind to purified detergentsolubilized la molecules while others do not. The peptides which bind la are those which, in combination with I-A or I-E, stimulate T-cell clones. Unlabeled peptides which competed for function competed for binding. Moreover, Grey reported that unrelated peptides which bound to the same la molecule could compete with one another. On the other hand, by photoaffinity labelling cell surface la with antigen, T. Delovitch and L. Phillips (Toronto) found that antigen could bind to la independently of H-2 haplotype. Thus, the degree of specificity with which antigen binds to la remains uncertain. T. Watts (Stanford) detected energy transfer between donor fluorescein-labelled peptide and acceptor Texas-red-labeled purified I-A in planar membranes but only in the presence of specific T cells. This result suggests that T cells are necessary to stabilize the interaction between the la molecule and antigen, and is consistent with the low affinity of binding between the la molecule and antigen fragment reported by Unanue and Grey (10 s106 M -1) but not with the higher affinities Onthe cell surface (109M-1)
reported by Delovitch. If the low affinity interaction in solution applies to the antigenpresenting cell (APC) surface, it is probable that other mechanisms exist to stabilize the antigen fragments on the surface of the APC. Studies by B. Benacerraf and K. Rock (Boston) indicate that the lipid membrane may contribute to stable binding of la and antigen. In their studies, APC treated with phospholipase A2 after pulsing with antigen shed the antigen and no longer presented that antigen but maintained their ability to present a second antigen introduced later as well as their allostimulatory ability. The lipase may cleave a covalent link between antigenic peptide and lipid formed during processing, or may disrupt a noncovalent association between peptide and membrane. The latter would be consistent with the results presented by J. Berzofsky (Bethesda) which showed that the ability of protein antigenic peptides to fold into stable amphipathic helices is important for antigen presentation and T-cell recognition. These amphipathic peptides may stably intercalate into the membrane at a concentration high enough to bind la despite the low affinity. Other cell surface molecules may also influence antigen presentation. E. Lakey (Chicago) described a molecule (72-74 kDa doublet) isolated from the surface of B cells by adsorption to an affinity column of a 23 amino acid peptide from cytochrome c. Polyclonal antibodies to this molecule inhibited peptide-specific T-cell proliferation. Another peptide-binding structure on presenting cells which enhanced antigen presentation was reported by S. Betancourt (Ann Arbor).
Protein antigenic structure A number of groups have been analysing the structural features of protein antigens which influence their stimulatory capabilities. Unlike B cells which recognize tertiary structure of proteins, T cells tend to recognize primary and maybe secondary structure of proteins. In a number of systems, T-cell responses to limited regions of the protein molecules are being analysed. B. Fox (Bethesda) showed that alterations ~n the C-terminal fragment of moth cytochrome c at position 99, which is thought to be the site recognized by the T cells, dramatically altered the
stimulatory capacity of the peptide. Alterations in position 103, thought to be the major contact site with la, affected heteroclicity. Whether one immunized with the natural peptide, containing lysine at position 103, or the variant containing asparagine at position 103, the peptide containing lysine stimulated more effectively. K. Cease (Bethesda) observed fine specificity differences between two Tcell clones specific for the same immunodominant myoglobin 102-118 peptide, detecting them by examining the rank order of potency of smaller peptides. These results and related observations by Grey for ovalbumin, by Unanue and by E. Sercarz (Los Angeles) for lysozyme and by A. Livingstone (Stanford) for myoglobin, suggest that an immunodominant site is the focus of a polyclonal response to a given protein molecule. It was suggested that immunodominance may be dependent on general structural features of the peptide. One relevant feature may be the ability of a peptide to fold into an amphipathic helix as suggested by Berzofsky. The T-cell receptor may generally interact with hydrophilic sites, whereas the la molecule or other structures on the APC may interact with hydrophobic sites on the peptide. Berzofsky showed that 18 of 23 known immunodominant helper T-cell sites can form amphipathic helices. Further analysis of the secondary structure of the isolated peptide will permit a more definitive answer. An interesting case of molecular mimicry was reported by A. Rees (London). An internal image anti-idiotype to a monoclonal antibody against a 38 kDa Myeobacterium tuberculosis protein stimulated a T-cell clone specific for the 38 kDa protein. Denaturation with loss of antigen binding did not affect the potency for stimulating T cells. Thus, the internal image protein may be processed like an exogenous antigen. Although T cells usually recognize antigen in conjunction with selfMHC molecules some T cells were said to be activated by antigen alone. D. Altman (Rehovot) reported that some antigen-specific T cells proliferate in response to native antigen in the absence of APC. This response was not inhibited by anti-la or anti-L3T4 antibodies and correlated with the ability of radiolabeled antigen to bind to the T cells. Antigen immobilized on 'Sepharose'
Immunology Today, voL 7, No. 17, 1986
beads also induced T-cell clone proliferation (M. Solvay, Ann Arbor). J. Klein (TObingen) showed that liposomes bearing a high density of antigen could stimulate T cells without la, whereas at lower antigen density, these liposomes also needed to bear la antigens. Anti-la blocked binding only when la was present, so uptake and presentation by any contaminating la ÷ cell was unlikely. One of the roles of la may be to present antigen to T cells in a more immunogenic form, so that T cells can bind the antigen with sufficient affinity to induce their activation. Some T cells may have a sufficiently high affinity for antigen to require only antigen for activation. These experiments argue against the extreme possibility that T cells recognize a neoantigen formed by the interaction of antigen and la molecules. APC must express la molecules but it is not clear whether la expression ensures a cell's capacity to present antigen. For example, it was first believed that activated but not resting B cells could present antigen. However under appropriate conditions, such as after low dose irradiation or mitomycin C treatment, resting B cells do present antigen, although less effectively than activated B cells. Benacerraf, and F. Manca and F. Celada (Genoa), and M. Watanabe and N. Hozumi (Toronto) showed that B cells present the antigen for which they were specific at concentrations up to 10 3fold lower than they could present other antigens. Moreover, Manca and Celada suggested that the specificity of the surface immunoglobulin on the presenting B cells also influences the specificity of T cells activated. Other studies presented at this meeting suggest that the structure of the la molecule on resting and activated B cells is different (D. Jenis, Denver). Liposomes which had incorporated la molecules from activated B cells were more efficient than liposomes containing resting Bcell la. Several studies indicated that some T-cell clones are unable to recognize B cells, regardless of their activation state (T. Kakiuchi, Tokyo; D. Umetsu, Boston; H. Nariuchi,
Tokyo). Different subsets of helper T together, these fragments formed cells may recognize different cells as an amphipathic structure. However, APC. T-cell clones that did not re- Sercarz presented evidence that spond to activated B cells could re- even in cases where an isolated pepspond to resting B cells (A. Finnegan, tide of lysozyme contained a comBethesda). Phorbol myristate acetate plete antigenic site, amino acids far could restore the T-cell response to • away in the sequence influenced activated B cells, suggesting that whether or not that site was seen activated B cells may not provide all when the native molecule was put in the appropriate signals for T-cell pro- culture or used to immunize. He liferation. Kakiuchi reported that suggested that these distant sites although certain T-cell clones did not influenced processing, and that this proliferate in response to B cells, effect could explain some of the they produced interleukin 2 and en- examples of an apparent influence larged. of tertiary structure on T-cell specificity. Delovitch described electron micAntigen processing roscopic evidence for antigen uptake Finally, the role of antigen proces- in endosomes, passage through the sing in antigen presentation was ex- Golgi apparatus, and re-expression tensively debated. Klein argued that on the cell surface. B. Askonas (Lonthe circumstantial evidence that pro- don) with A. Townsend and A. teolytic antigen degradation was re- McMichael (Oxford), indicated that quired could be explained in other although CTLs specific for influenza ways. The agents used to inhibit nucleoprotein could see small synthprocessing, such as chloroquine and etic peptides on target cells, convenglutaraldehyde, have multiple effects tional inhibitors of processing had no and, furthermore, do not inhibit, effect. Other pathways of processing antigen presentation consistently. If may be active for these proteins. antigen were cleaved in acidic lyso- Grey and Berzofsky presented evisomes or endosomes and peptides dence that proteolysis is not always recycled with ia and re-expressed on necessary. For instance, unfolding of the cell surface bound to la, one the intact protein was shown to be would have to postulate that the sufficient in several cases. There may interaction between peptide and la be more than one means to the was stable at both the low pH of the same end, but physiologically the cell endosomes and the neutral pH at most frequently uses proteolysis. The the cell surface. Klein presented evi- controversy may be resolved by dence that native proteins, reduced accepting a broader view, that proand covalently coupled to lipid by a cessing is more than just proteolysis. disulfide bond and incorporated into If the purpose of processing is to liposomes, could be presented to T expose residues needed for interaccells without APC to perform pro- tion with the TCR or the APC, unteolytic processing. Also, K. Ziegler folding the molecule or attaching it (Atlanta) reported a high molecular to a lipid to anchor it in the lipid weight protein of Listeria which bilayer may accomplish the same appeared to be presented without goal without proteolysis. These processing. Klein also cited examples molecules would have been artificialin which protein tertiary structure ly processed, and no longer native• appeared to influence T-cell recog- However, even the generalization nition. D.B. Thomas (London) pre- that native protein antigens require sented another such example for in- some alteration may apply only to fluenza hemagglutinin. Delovitch the class of exogeneous wateralso demonstrated a T-cell recogni- soluble globular proteins: intrinsic tion site which required parts of both membrane proteins, for example the A and B chain of insulin, may be handled differently. although in this case the fragments were covalently coupled via a dis- 7Immunology Branch and 2Metabolism Branch, National Institutes of Health, National ulfide bond, so tertiary conformation was not necessarily critical. Indeed, Cancer Institute, Bethesda, MD 20892, USA
0
319
TOROI
.TCHBOOK
In July over 700 delegates gathered in Toronto for the 6th International Congress of Immunology. In 5 days they were given an opportunity of attending 6 plenary lectures and, in the parallel sessions, 110 symposium talks +and 1100 workshop presentations. To review thes papers comprehensiw _
~ould be impractical and of dubious value (readers can expect, in any case, that important topics will continue to be reviewed at adequate length in issues of this journal). What follows is a collection of invited commentaries on some aspects of immunology in 1986, as reported at the Congress and certain satellite meetings. The correspondents were asked to select from the papers they heard what they thought was stimulating. They were brief, at the Editor's insistence, and so inevitably had to omit from their reports some r4ork worthy of attention. ~ _ _
This report attempts to highlight the new findings presented at the Congress on T-cell recognition of antigen, concentrating on the T-cell receptor, antigen processing and presentation, and the types of antigenic structures recognized by T cells. We do not attempt to cover other related topics such as the ontogeny of T cells or their receptors, or the regulatory function of T cells. The major histocompatibility complex (MHC)-restricted receptor on antigen-specific T cells is a disulfidelinked heterodimer composed of two subunits (e and 13)of 40-50 kDa each. There has been remarkable progress in the molecular analysis of the c~ and 13 genes encoding the T-cell receptor (TCR) over the past few years. In addition to the e and 13 genes, a third rearranged TCR gene, T~, has been identified. The identification of a protein molecule which could be the product of the T-/ gene was reported by M. Brenner (Boston). A population of T cells was found to express surface T3 but neither surface TCR c~nor 13. It lacked mRNA encoding TCR ct and 13chains but expressed T~/ mRNA. Chemical crosslinking and immunoprecipitation with anti-T3 identified two new nondisulfide-linked proteins of 55 kDa and 40 kDa. In addition, antisera raised against synthetic peptides rep~) 1986, Elsevier Science Publishers B.V., Amsterdam
Antigenrecognitionby T cells Alison Finnegan and JayA. Berzofsky2 resenting 17 amino acids in the V~/ gene and a 20 amino acid stretch in the C~ region immunoprecipitated the 55 kDa molecules from the T3+TCR - cells. Thus the 55 kDa molecule may be the protein encoded by the T~ gene. The additional molecule of 40 kDa is designated Ta. These two polypeptides may form a T3-associated, nondisulfide-linked heterodimer and a second TCR. Parallel results were reported by L. Chess (New York) examining a cloned T3+T4-T8 - human thymocyte cell line. This T-cell clone lacks expression of mature mRNA for c~ and 13 chains but expresses T-y mRNA, and precipitation with antiT3 identified a nondisulfide-linked complex that includes, in addition to T3, two new proteins of 44 kDa and 62 kDa. Antisera to a synthetic peptide corresponding to a portion of the constant region of the T~/ gene precipitated the 44 kDa molecule. A similar structure that could represent the -y gene product was reported by S. Tonegawa (Cambridge, M A ) i n L3T4- Lyt 2 thymocytes which express ~/mRNA but lack TCR ~ and 13 mRNA. Also, T. Mak (Toronto) re-
0167 - 4919186/$02.00
ported ~/mRNA expression in nylonpassed cells of nude mice which express neither c~ nor 13 transcripts. Thus, the ~I chain seems finally to have been found, but on cell populations which do not express the c~13 TCR. The function of these cell populations and of the receptor they express is not yet known. The relative contributions of the and 13 chains of the TCR in the recognition of MHC and antigen is poorly understood. F. Rupp (Zurich) demonstrated that two T-cell clones of different antigen and MHC specificities share the same idiotype and express the same V~ and V13. The two clones differ in the D-J(13) portion of the TCR 13 chain, so the D-J region may account for the differences in specificity of these T-cell clones. In addition, A. Blackman (Denver) demonstrated that determinants formed by the combination of and 13 chains contribute to the specificity of both chains. Interestingly, J. Kappler (Denver) reported an influence of H-2 haplotype on the usage of a particular V13 family (V138) which may reflect the effect of H-2 on T-cell specificities in the reper- 317
0
Immunology Today, vol. 7, No. 1 I, 1986
toire. Cell surface structures other than the TCR may be important for some T-cell clone specificity. The (~ and 13 genes from a cytotoxic T-cell (CTL) clone were transfected into another CTL clone as reported by S. Tonegawa but expression of these TCR chains was not sufficient to endow the recipient clone with the specific cytotoxicity of the donor clone. The ability to express and isolate V(~ and VI3 in soluble form attached to Ig constant regions, as described by M. Davis (Stanford), may ultimately allow the resolution of these questions by direct binding studies.
0
P.
318
Antigen-la interactions T helper/inducer cells usually recognize exogenous antigen in association with MHC class II molecules. In the mouse, cloned T cells selectively recognize different antigens in association with either the I-A or I-E class II molecules. The determinant selection hypothesis states that there is specificity to the interaction between the antigen and la molecules in the formation of a complex recognized by the T cells. E. Unanue (St. Louis) and H. Grey (Denver) showed by equilibrium dialysis that certain peptides bind to purified detergentsolubilized la molecules while others do not. The peptides which bind la are those which, in combination with I-A or I-E, stimulate T-cell clones. Unlabeled peptides which competed for function competed for binding. Moreover, Grey reported that unrelated peptides which bound to the same la molecule could compete with one another. On the other hand, by photoaffinity labelling cell surface la with antigen, T. Delovitch and L. Phillips (Toronto) found that antigen could bind to la independently of H-2 haplotype. Thus, the degree of specificity with which antigen binds to la remains uncertain. T. Watts (Stanford) detected energy transfer between donor fluorescein-labelled peptide and acceptor Texas-red-labeled purified I-A in planar membranes but only in the presence of specific T cells. This result suggests that T cells are necessary to stabilize the interaction between the la molecule and antigen, and is consistent with the low affinity of binding between the la molecule and antigen fragment reported by Unanue and Grey (10 s106 M -1) but not with the higher affinities Onthe cell surface (109M-1)
reported by Delovitch. If the low affinity interaction in solution applies to the antigenpresenting cell (APC) surface, it is probable that other mechanisms exist to stabilize the antigen fragments on the surface of the APC. Studies by B. Benacerraf and K. Rock (Boston) indicate that the lipid membrane may contribute to stable binding of la and antigen. In their studies, APC treated with phospholipase A2 after pulsing with antigen shed the antigen and no longer presented that antigen but maintained their ability to present a second antigen introduced later as well as their allostimulatory ability. The lipase may cleave a covalent link between antigenic peptide and lipid formed during processing, or may disrupt a noncovalent association between peptide and membrane. The latter would be consistent with the results presented by J. Berzofsky (Bethesda) which showed that the ability of protein antigenic peptides to fold into stable amphipathic helices is important for antigen presentation and T-cell recognition. These amphipathic peptides may stably intercalate into the membrane at a concentration high enough to bind la despite the low affinity. Other cell surface molecules may also influence antigen presentation. E. Lakey (Chicago) described a molecule (72-74 kDa doublet) isolated from the surface of B cells by adsorption to an affinity column of a 23 amino acid peptide from cytochrome c. Polyclonal antibodies to this molecule inhibited peptide-specific T-cell proliferation. Another peptide-binding structure on presenting cells which enhanced antigen presentation was reported by S. Betancourt (Ann Arbor).
Protein antigenic structure A number of groups have been analysing the structural features of protein antigens which influence their stimulatory capabilities. Unlike B cells which recognize tertiary structure of proteins, T cells tend to recognize primary and maybe secondary structure of proteins. In a number of systems, T-cell responses to limited regions of the protein molecules are being analysed. B. Fox (Bethesda) showed that alterations ~n the C-terminal fragment of moth cytochrome c at position 99, which is thought to be the site recognized by the T cells, dramatically altered the
stimulatory capacity of the peptide. Alterations in position 103, thought to be the major contact site with la, affected heteroclicity. Whether one immunized with the natural peptide, containing lysine at position 103, or the variant containing asparagine at position 103, the peptide containing lysine stimulated more effectively. K. Cease (Bethesda) observed fine specificity differences between two Tcell clones specific for the same immunodominant myoglobin 102-118 peptide, detecting them by examining the rank order of potency of smaller peptides. These results and related observations by Grey for ovalbumin, by Unanue and by E. Sercarz (Los Angeles) for lysozyme and by A. Livingstone (Stanford) for myoglobin, suggest that an immunodominant site is the focus of a polyclonal response to a given protein molecule. It was suggested that immunodominance may be dependent on general structural features of the peptide. One relevant feature may be the ability of a peptide to fold into an amphipathic helix as suggested by Berzofsky. The T-cell receptor may generally interact with hydrophilic sites, whereas the la molecule or other structures on the APC may interact with hydrophobic sites on the peptide. Berzofsky showed that 18 of 23 known immunodominant helper T-cell sites can form amphipathic helices. Further analysis of the secondary structure of the isolated peptide will permit a more definitive answer. An interesting case of molecular mimicry was reported by A. Rees (London). An internal image anti-idiotype to a monoclonal antibody against a 38 kDa Myeobacterium tuberculosis protein stimulated a T-cell clone specific for the 38 kDa protein. Denaturation with loss of antigen binding did not affect the potency for stimulating T cells. Thus, the internal image protein may be processed like an exogenous antigen. Although T cells usually recognize antigen in conjunction with selfMHC molecules some T cells were said to be activated by antigen alone. D. Altman (Rehovot) reported that some antigen-specific T cells proliferate in response to native antigen in the absence of APC. This response was not inhibited by anti-la or anti-L3T4 antibodies and correlated with the ability of radiolabeled antigen to bind to the T cells. Antigen immobilized on 'Sepharose'
Immunology Today, voL 7, No. 17, 1986
beads also induced T-cell clone proliferation (M. Solvay, Ann Arbor). J. Klein (TObingen) showed that liposomes bearing a high density of antigen could stimulate T cells without la, whereas at lower antigen density, these liposomes also needed to bear la antigens. Anti-la blocked binding only when la was present, so uptake and presentation by any contaminating la ÷ cell was unlikely. One of the roles of la may be to present antigen to T cells in a more immunogenic form, so that T cells can bind the antigen with sufficient affinity to induce their activation. Some T cells may have a sufficiently high affinity for antigen to require only antigen for activation. These experiments argue against the extreme possibility that T cells recognize a neoantigen formed by the interaction of antigen and la molecules. APC must express la molecules but it is not clear whether la expression ensures a cell's capacity to present antigen. For example, it was first believed that activated but not resting B cells could present antigen. However under appropriate conditions, such as after low dose irradiation or mitomycin C treatment, resting B cells do present antigen, although less effectively than activated B cells. Benacerraf, and F. Manca and F. Celada (Genoa), and M. Watanabe and N. Hozumi (Toronto) showed that B cells present the antigen for which they were specific at concentrations up to 10 3fold lower than they could present other antigens. Moreover, Manca and Celada suggested that the specificity of the surface immunoglobulin on the presenting B cells also influences the specificity of T cells activated. Other studies presented at this meeting suggest that the structure of the la molecule on resting and activated B cells is different (D. Jenis, Denver). Liposomes which had incorporated la molecules from activated B cells were more efficient than liposomes containing resting Bcell la. Several studies indicated that some T-cell clones are unable to recognize B cells, regardless of their activation state (T. Kakiuchi, Tokyo; D. Umetsu, Boston; H. Nariuchi,
Tokyo). Different subsets of helper T together, these fragments formed cells may recognize different cells as an amphipathic structure. However, APC. T-cell clones that did not re- Sercarz presented evidence that spond to activated B cells could re- even in cases where an isolated pepspond to resting B cells (A. Finnegan, tide of lysozyme contained a comBethesda). Phorbol myristate acetate plete antigenic site, amino acids far could restore the T-cell response to • away in the sequence influenced activated B cells, suggesting that whether or not that site was seen activated B cells may not provide all when the native molecule was put in the appropriate signals for T-cell pro- culture or used to immunize. He liferation. Kakiuchi reported that suggested that these distant sites although certain T-cell clones did not influenced processing, and that this proliferate in response to B cells, effect could explain some of the they produced interleukin 2 and en- examples of an apparent influence larged. of tertiary structure on T-cell specificity. Delovitch described electron micAntigen processing roscopic evidence for antigen uptake Finally, the role of antigen proces- in endosomes, passage through the sing in antigen presentation was ex- Golgi apparatus, and re-expression tensively debated. Klein argued that on the cell surface. B. Askonas (Lonthe circumstantial evidence that pro- don) with A. Townsend and A. teolytic antigen degradation was re- McMichael (Oxford), indicated that quired could be explained in other although CTLs specific for influenza ways. The agents used to inhibit nucleoprotein could see small synthprocessing, such as chloroquine and etic peptides on target cells, convenglutaraldehyde, have multiple effects tional inhibitors of processing had no and, furthermore, do not inhibit, effect. Other pathways of processing antigen presentation consistently. If may be active for these proteins. antigen were cleaved in acidic lyso- Grey and Berzofsky presented evisomes or endosomes and peptides dence that proteolysis is not always recycled with ia and re-expressed on necessary. For instance, unfolding of the cell surface bound to la, one the intact protein was shown to be would have to postulate that the sufficient in several cases. There may interaction between peptide and la be more than one means to the was stable at both the low pH of the same end, but physiologically the cell endosomes and the neutral pH at most frequently uses proteolysis. The the cell surface. Klein presented evi- controversy may be resolved by dence that native proteins, reduced accepting a broader view, that proand covalently coupled to lipid by a cessing is more than just proteolysis. disulfide bond and incorporated into If the purpose of processing is to liposomes, could be presented to T expose residues needed for interaccells without APC to perform pro- tion with the TCR or the APC, unteolytic processing. Also, K. Ziegler folding the molecule or attaching it (Atlanta) reported a high molecular to a lipid to anchor it in the lipid weight protein of Listeria which bilayer may accomplish the same appeared to be presented without goal without proteolysis. These processing. Klein also cited examples molecules would have been artificialin which protein tertiary structure ly processed, and no longer native• appeared to influence T-cell recog- However, even the generalization nition. D.B. Thomas (London) pre- that native protein antigens require sented another such example for in- some alteration may apply only to fluenza hemagglutinin. Delovitch the class of exogeneous wateralso demonstrated a T-cell recogni- soluble globular proteins: intrinsic tion site which required parts of both membrane proteins, for example the A and B chain of insulin, may be handled differently. although in this case the fragments were covalently coupled via a dis- 7Immunology Branch and 2Metabolism Branch, National Institutes of Health, National ulfide bond, so tertiary conformation was not necessarily critical. Indeed, Cancer Institute, Bethesda, MD 20892, USA
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