viewpoint

NK cells and T cells: mirror images? Rogier Versteeg The expression of MHC class I molecules protects cells against lysis by natural killer (NK) cells. It is possible that NK cells are 'educated' to recognize self MHC class I molecules and that the combination of self peptide and MHC class I molecule blocks NK-mediated lysis. Here, Rogier Versteeg compares and contrasts models of education and self-nonself discrimination by T cells and NK cells, and presents a hypothesis for the evolution of T cells from NK cells. The idea that major histocompatibility complex (MHC) class I expression protects against natural killer (NK) lysis 1-3 has been confirmed recently by gene transfection experiments 4-6. Transfection of human leucocyte antigen (HLA) class I genes into an HLA class-I-negative mutant cell line restored the NK-resistant phenotype. However, while HLA-A3, HLA-Aw68, HLA-B7, HLA-B27 and HLA-Bw58 protected against NK lysis, HLA-A2 did not 7. Analysis of recombinant genes of HLA-A2 and HLA-Aw68 showed the inability of HLA-A2 to protect stems from the presence of a histidine residue at position 74 in the c~1 domain 8. Site-directed mutagenesis of His74 to Asp74 (as in HLA-Aw68) rendered HLA-A2 protective to NK lysis 8. The His74 in HLA-A2 blocks a pocket in the groove that is accessible in HLA-Aw68 (Ref. 9). This indicates that protection against NK lysis is provided by a peptide that protrudes into the Asp74 pocket of the HLA class I groove. The addition of synthetic viral peptides to target cell cultures increases their susceptibility to NK lysis (W.J. Storkus and J.R. Dawson, unpublished), suggesting that viral peptides can displace a protective self peptide to render the cells susceptible to NK lysis. Addition of certain synthetic self peptides also increases NK susceptibility 1°, indicating that protection is conferred by a particular self peptide, or group of self peptides, that have to compete for presentation with other, nonprotective, self peptides. Further support for this idea comes from RMA-S cells. These cells cannot load their class I MHC molecules with peptidel~; consequently, they lack MHC class 1 cell surface expression and are prone to NK lysis. Addition of exogenous peptide restores MHC class I molecule expression, but does not convert the cells to the NKresistant phenotype ~2. This again suggests that only specific peptides can protect against NK lysis. Education of NK cells There are indications that NK cells in mice are educated to recognize self MHC class I molecules. The RBL-5 lymphoma cell line is oncogenic in syngeneic C57BL/6 (H-2 b) mice, and RBL-5 variants that lack MHC class I expression are rejected by NK cells. However, C57BL/6 mice that express H-2D d as a transgene also reject normal RBL-5 cells 13,14. This suggests that some clones of NK cells in the transgenic mice are educated to recognize the H-2D d as self, and that they kill RBL-5 cells that have quantitatively normal MHC class I expression, but lack one self MHC specificity.

Selection of NK cells could also occur in humans. Moretta and coworkers have cloned human CD3- NK cells that kill normal cells from some allogeneic individuals. NK cells of different specificity were cloned from one individual and, when these were tested against a panel of allogeneic targets, four specificities could be discerned ~5. Targets from a large kindred were tested for susceptibility to one of these clones: resistance to killing was dominantly inherited and cosegregated with the MHC class I region 16, suggesting that a particular MHC class I allele protects against the NK clone. Two new molecules, of 55-58 kDa, have been identified on the NK clones. They are associated with the CD3 ~-chain and are almost identical, but display some unique sequences (A. Moretta, unpublished). Phenotyping the NK clones with monoclonal antibodies (mAbs) to these molecules gives four patterns of staining: +/+, + / - , - / + or - / - . The staining patterns match exactly to the four specificities of the clones ~7, raising the possibility that the mAbs recognize polymorphic structures that determine the specificity of the NK cells. Interestingly, individuals susceptible to NK clones of the ' + / - ' phenotype are themselves virtually devoid of such NK cells w. This indicates that the NK cell population is subject to selection against potentially autoreactive clones. Selection of NK cells A tentative scheme of NK selection and specificity can now be drawn, based on the studies described above. NK precursors may express an antigen receptor in a clonally variable form, resulting from alternative gene expression, alternative splicing or intragenic recombination. The precursors are then selected for high-affinity binding to MHC class I plus protective peptide (Fig. 1). This could take place in a compartment in which MHC class I molecules exclusively present protective peptides. Alternatively, protective peptides could be a class of abundant self peptides, and NK precursors are selected on the basis of avid binding to a large number of MHC class I molecules per cell in the selective compartment. Precursors with low affinity for MHC class I plus protective peptide are deleted or rendered anergic. Selected cells mature and develop the effector function 'recognition - no kill; no recognition -0 kill'. After arrival in the circulation, interaction with target cells that did not express the complex of MHC class I plus protective peptide would induce NK cell lysis, while recognition of the complex would prevent NK cell activation.

© 1992, Elsevier Science Publishers Ltd, UK.

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viewpoint ~ By this model, cells with abnormal MHC class I expression or insufficient protective peptide expression, such as tumor cells or virally infected cells, arc prone to lysis. Many tumors express high levels of the c-myc or N-myc oncogenes, which switch off class I MHC expression 18a9. The c-myc oncogene downmodulates HLA-B expression in melanomas 2°, rendering these cells susceptible to NK lysis (Ref. 21 and L.T.C. Peltenburg and P.l. Schrier, unpublished). In many other tumors the expression of MHC class [ alleles is abrogated by as yet unknown mechanisms. In virally infected cells, the abundantly produced viral peptides could compete out the protective self peptide, rendering the cells susceptible to lysis.

f/

Precursor \,

Expression of variable receptor // / ///

,\

@ Highaffinity

Presentation of MHC molecule plus protective peptide

LOW-

affinity I

V

Protective peptide

Effector function Recognition: no kilt No recognition: kill

The possibility that NK cells recognize the complex of MHC class I plus protective peptide raises the intriguing question of why T cells and NK cells recognize similar target structures and what (if any) evolutionary relationship exists between the two cell types. According to the model presented above, a good proportion of the MHC class I molecules on somatic cells must present protective peptides to avert NK lysis. The self peptides most frequently presented by HLA-B27 were recently identified 22. Most of them appear to be derived from proteins that are highly conserved in evolution: heat shock protein 90 (hsp90), histone H3, elongation factor 2 (EF2) and two ribosomal proteins. The presented peptides themselves are exceptionally well conserved from yeast to humans (Table 1). Although there is no evidence that this abundant class of conserved peptides do function as protective peptides, some interesting speculations can be made. Why would primitive NK cells have evolved to recognize conserved peptides? The answer could be that the conserved peptides encoded by infectious agents are virtually identical to their mammalian homologues. Upon infection with a parasite, peptides from the parasite will compete with self peptides for presentation by MHC

Deleted or inactivated

Natural killer cell Fig. 1. Model fi)r selectkm of NK cells. molecules. Peptides derived from abundantly produced parasite hspg0 are very similar, but not identical to endogenous hsp90 peptides, so they represent the most efficient way of competing with self bsp90-derived peptides (see Table 1). The small differences, however, could prevent recognition of the complex of MHC plus peptide by the NK receptor, rendering an infected cell susceptible to NK lysis. Thus, recognition of conserved peptides by NK cells could provide an excellent strategy for the recognition of infected cells. From NK cell to primitive T cell: through the looking glass If hsps (for brevity, hsps denotes the whole group of conserved proteins) do function as protective peptides, the 'NK receptor' gene(s) should encode a repertoire that

Table 1. Comparison of predominant peptides found in the HLA-B27 groove 23 with their homologues in other organisms Peptide source

Amino acid position

Organism

1

2

3

4

5

6

7

8

9

Ribosomal protein

G G G

R R R

i i i

D D D

K K K

P P P

i m i

L L L

K K K

Human Slime mold Yeast

Elongation factor 2

R R R

R t k

W W f

L L L

P P P

A A A

G G a

D e D

A

A A

Human Drosophila Slime mold

R

R

i

K

E

i

V

K

s k

k R

i i

K K

E E

i v

V i

n K

K K r

Human l)rosophila Yeast

R

R

Y

Q

K

S

T

E

L

R

R

f

Q

K

S

T

E

L

Human Yeast

245

V,,l

N o 7 t ge

HSP90a

Histone H3

Immunology Today

viewpoint covers all potential combinations of MHC alleles and hsp alleles in a population. Then, only NK precursors with a receptor that binds with high affinity to self MHC molecules carrying self hsp-derived peptide will develop the

Precursor

/ \ Expression of variable receptor

\

/

@-@-a Presentation of MHC molecules plus conserved peptide

Highaffinity

Lowaffinity

1 Effector function Recognition: no kill No recognition: kill

I Effector function No recognition: no kill Recognition: kill

1 Natural killer cell

1 Primitive T cell

effector function: ‘recognition + no kill; no recognition -+ kill’. Since hsps are extremely conserved in evolution, it is likely that among the nonselected precursor cells many have receptors capable of recognizing combinations of self MHC and nonself hsp. The most primitive T cells could have evolved from this unused end of the NK selection system. If a nonselected precursor, instead of being inactivated, acquired the inverted effector function ‘no recognition + no kill; recognition - kill’, then a cell with the essential characteristics of a T cell would have been born (Fig. 2). Such cells would not kill normal autologous cells, because they are not recognized by the antigen receptor. However, autologous cells infected by a parasite and exposing parasite-encoded hsp in the MHC groove might be recognized and destroyed by some of the new T-cell-like effecters. The advantage of this system is that cells displaying only minor amounts of nonself peptide can be killed. In this model, NK receptor selection by polymorphic MHC molecules pre-dates the evolution of T cells. This is possible as protochordate ascidians have specialized cells that recognize a polymorphic self antigen23. These organisms can readily fuse, but reject partners that are not fully identical for a multiallelic locus. This prevents fusion between genetically nonidentical cells, distinguishing genetically homogeneous individuals from genetically heterogeneous colonies. The similarity with mammalian rejection of grafts lacking self MHC molecules is striking, but the molecular determinants of the ascidians await identification.

Fig. 2. Model for a primitive NK cell and T-cell selection system.

Precursor

0

,

‘1

I/

Rearrangement

,

of T-cell receptor

&/*;b

1 Highaffinity

Presentation of MHC molecules plus educating peptide

t Intermediate or low-affinity ,I

Ieleled or anergic

,’

/

\ ..~

‘\

\;__~~~~~~

--~----&-&_$ZI’

1 Highaffinity

DelLted

Presentation of MHC molecules plus ‘random’ self peptides

t Lowaffinity

c Effector function No recognition: no ki Recognition: kill ~~___~_~~__~_~~ c Peripheral T cell

Fig. 3. A model for T-cell selection in the thymus.

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Are modern T cells selected by archaic molecules? Undoubtedly, present-day T cells use a more sophisticated receptor than NK cells. If they evolved from NK cells, their ontogeny should probably still reflect this relationship. Some aspects of c@ T-cell differentiation and selection are generally appreciated. First, precursor T cells with affinity for self MHC molecules are positively selected in the cortex of the thymus24,25. In the medulla they are subsequently negatively selected for recognition of self peptides presented by self MHC molecules. It has been proposed that the thymic cortical epithelium only selects T cells with weak to intermediate affinity for self MHC, while precursors with high-affinity receptors are deleted 26-2B(Fig. 3). Kappler and Marrack have proposed that the positive selection in the cortex is performed by ‘educating’ peptides that exclusively occupy the grooves of cortical MHC molecules”. The combination of self MHC and ‘educating’ peptide would thus shape the repertoire of the c@ TCR. One possibility is that the ‘educating’ peptides used in this early cortical selection step are still the ‘protective peptides’ (Fig. 3). If so, they bear heavily on the selection of the TCR repertoire. In support of this possibility, the TCR repertoire shows a strong bias to the recognition of heat shock proteins30J1. The rationale of such a selection would be that TCRs that interact weakly with self MHC molecules plus conserved self peptides have a high probability of strong recognition of self MHC plus conserved nonself peptide. It was recently proposed that the repertoire of the TCR is directed to a limited set of microbial antigens that mimic self antigens32. This repertoire forms the

Vol. 13 NO. 7 1992

viewpoint immunological homunculus', the immune system's reflection of self that defines nonself. The model of evolution of T cells as a mirror image of NK cells predicts a bias in the repertoire that, by its knowledge of self, can detect nonself.

I. Exp. Med. 168, 1469-1474

14 H6glund, P., (;las, R., Ohlen, C., Ljunggren, H-G. and Kiirrc, K. (1991)/. Exp. Med. 174, 327-334 15 Ciccone, E., Moretta, A. and Moretta, L. (1992) hnmunol. Lett. ,31, 99-104 16 Ciccone, E., ()donna, M., Viale, O. et al. (1990) Proc. Natl Acad. 5ci. USA 87, 9794-9797; (199 I) Prec. Natl Acad. Rogier Versteeg is at the Dept of Human Genetics, Sci. USA 88, 5477 (Correction) Academic Medical Centre, University of Amsterdam, 17 Moretta, A., Bottino, C., Pende, D. et al. (1990)I. l~xP. Meibergdreef 15, 1105 A Z Amsterdam, The Netherlands. Med. 172, 1589-1598 18 Versteeg, R., Noordermeer, 1.A., Krfise-Wolters, M., References Ruiter, D.J. and Schrier, P. (1988) EMB()J. 7, 1(/23-1029 1 Ljunggren, H-G. and Kfirre, K. (1990) lmmunol. Today 11, 19 Bernards, R., Dessam, S.K. and Weinberg, R.A. (1986) 237-244 Cell 46, 667-674 2 Snell, G.D. (1976) Transplant. Proc. 8,147-156 20 Versteeg, R., Krfise-Wolters, K.M., Plomp, A.C et al. 3 K~irre,K., Llunggren, H.G., Piontek, G. and Kiesling, R. (1989) J. Exp. Med. 170, 621-635 (1985) Nature 319,675-678 21 Versteeg, R., Pcltenburg, L.T.C., Plomp, A.C. and Schrier, 4 Quillet, A., Presse, F., MarchioI-Fournigault, C. et al. P.I. (1989).]. hnmunol. 143,4331-4337 (1988)J. hnmunol. 141, 17-20 22 Jardetzky, T.S., Lane, W.S., Robinson, R.A., Madden, 5 Shimizu, Y. and DeMars, R. (1989) Eur. J. Immunol. 19, D.R. and Wiley, D.C. (1991) Nature 353, ,:;26-329 447-451 23 Weissman, I.L., Saito, Y. and Rinkevich, B. (1990) 6 Storkus, W.J., Alexander, J., Payne, J.A., Dawson, J.R. lmmunol. Rev. [1 ~,, 227-241 and Cresswell, P. (1989) Proc. Natl Acad. Sci. USA 86, 24 Bevan, M. (19-7) Nature 269, 417-418 2361-2364 25 Zinkernagel, R., Callahan, A., Althage, A. et al. (1978) 7 Storkus, W.J., Alexander, J., Payne, J.A., Cresswell, P. and .]. Exp. Med. 147, 882-896 Dawson, J.R. (1989)J. Immunol. 143, 3853-3857 26 Sprent, J. and Webb, S.R. (1987) Adz,. hnmunol. 41, 8 Storkus, W.J., Salter, R.D., Alexander, J. et al. (1991) Proc. 39-13~ Natl Acad. Sci. USA 88, 5989-5992 27 Gao, E-K., Lo, D. and Sprent, 1. (1990)J. Exp. Med. 171, 9 Garrett, T.P.J., Saper, M.A., Bjorkman, P.J., Strominger, 1101-1121 J.L. and Wiley, D.C (1989) Nature 342,692-696 28 Vandekerckhove, B.A.E., Namikawa, R., Bacchetta, R. 10 Chadwick, B.S., Sambhara, S.R., Roder, J.C. and Miller, and Roncarolo, M-G. (1992)J. Exp. Med. 175, R.G. (1991) Nat. Immun. Cell Growth Regul. 10, 127 1033-1 {)43 (Abstr.) 29 Marrack, P. and Kappler, J. (I 988) hmnunol. Today 9, 11 Townsend, A., Ohlen, C., Bastin, J. et al. (1989) Nature 308-315 340, 443-448 30 Raulet, D.H. (1989) Nature 339, 342-343 12 Frankkson, L., Ohlen, C., H6glund, P., Ljunggren, H.G. 31 Winfield, J. and Jarjour, W. (1991) lmmunol. Rev. 121, and Kiirre, K. (1991) Nat. hnmun. Cell Growth Regul. 10, 193-220 122 (Abstr.) 32 Cohen, I.R. and Young, D.B. (1991) hnmunol. Today 12, 13 H6ghmd, P., Ljunggren, H-G., Ohten, C. et al. (1988) 105-110

Erratum In the Viewpoint article "Intermolecular cooperativity: a clue to why mice have IgG3 ?" by Neil S. Greenspan and Laurence J.N. Cooper, two errors were introduced.

(1) On p.165, column 1, line 9, the sentence "IgM-, IgGl-, IgG2b- and IgG3derived -F(ab')2 fragments..." should read "IgM, IgG1 and IgG2b mAbs and IgG3-derived F(ab') 2 fragments..."

(2)

On p. 167, legend to Fig. 2, "The cooperating antibodies would exhibit a higher relative specificity for an antigenic surface bearing two related epitopes versus..." should read "The cooperating antibodies would exhibit a higher relative specificity for an antigenic surface bearing both cognate epitopes versus a surface containing only one or the other of the two epitopes".

We apologize to the authors and to readers for these errors.

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Vol. 1,3 No. 7 1992