Sti uctiire m d Function of Clycosyl~itiori

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expression of (;A(; receptors. This study confirmed at the niolecular level the existence of a diverse assay of anionic carbohydrate-specific receptors on lymphocytes which are both cell and ligand specific. IJnfortunately, the majority of the GAG-binding molecules detected on lymphocytes did not correspond to known lymphocyte antigens [ 101. I Iowever, additional studies have shown that three, Mell-characterized, lymphocyte cell-surface molecules bind anionic polysaccharides, namely, C112, (‘114 and C1145. With CI)45 it appears that the thymic form of the molecule binds heparin [ 10 I and other SP [HI whereas extrathymic C114.5 does not I101. Since the three N-terminal exons ofCI14.5 can potentially produce eight distinct forms of the niolecule [ 11 ], it appears that the thymocyte form of CI 14.5 expresses a heparin/SP-binding site which is lost, possibly owing to different splicing, on the splenocyte form. ‘I’he possibility that anionic polysaccharides, particularly heparan sulphates, act as important functional ligands for CI14.5 is currently being investigated. The CI)2 molecule is an important adhesion molecule on ‘I’ lymphocytes that participates in the interaction of ‘I’ cells w ith antigen-presenting cells and target cells 11 1. Ikised on monoclonal antibody binding studies three epitopes have been identified on CD2, termed ‘1’1 1 ,, ‘1’1 1, and T11; [ 121. There is compelling evidence that a cell-adhesion ligand for the ‘1’1 1, epitope of CI12 is the widely distributed 55-70 kI)a glycoprotein, LFA-3 [ 11. Recently w e have observed that anionic polysaccharides, such as dextran sulphate. bind to CI)2 and the anionic sugar binding site maps to the 7’1 1, region of the molecule 11 3, 141. This finding has raised the possibility that there is an alternative ligand for C1>2 which is an anionic sugar, possibly a sulphated (;A(;. Studies in progress have provided preliminary evidence for the existence of two new celladhesion ligands for C1>2 but whether the new ligands are anionic carbohydrates requires further investigation. Another intriguing possibility being tested is that the anionic sugar binding site on CI)2 is not involved in cell-cell adhesion but interacts with G A G in extracellular matrices and basement membranes. In the case of CI)4, reports by us [ 151 and others [ l o , 171 indicate that the molecule can bind SP, such as dextran sulphate, and a range of other polyanions. Monoclonal antibody blocking studies have mapped the polyanion binding site to the two N-terminal, Ig-like, domains of the molecule [ 15 1. This binding site is distinct but closely associated with the HIV-gpl20 binding region of CD4 which

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can at least partially explain the anti-I IIV activity of some SP [ 1.5 I. The major function of C114, however, is to interact with class I1 major histocompatibility complex (MHC) molecules and aid activation of ‘I’ cells by peptide-class 11 MI IC complexes [18]. b e d on the recently solved threedimensional structure of the first two Ig-like domains of human CD4 [ 19, 201 it appears that the class I1 MI 1C- and anionic sugar-binding regions may overlap. 130th binding sites extend across the two Ig-like domains. with concentrations of basic residues in each domain aligning on one face of the molecule to potentially form an elongated polyanion-binding site. Such a binding site could readily accommodate a relatively large and linear anionic polysaccharide. ‘I‘he geometry of the polyanionbinding site may explain why only high-molecularmass ( > 10 kl)a) dextran sulphate binds to C1)4 I 1.51. On the other hand, very low-molecular-mass polyanions which bind to CD4, such as aurintricarboxylic acid, may interact at multiple points along the binding site. The close proximity of the class I1 MI IC- and polyanion-binding sites on CI)4 raises the possibility that CI 14 may recognize an anionic polysaccharide associated with class I1 MI I(’ molecules. This hypothesis is supported by the report that the invariant chain of class 11 MIiC carries a sulphated GAG side-chain which is essential for antigen presentation [ 2 1 , 22 1. Furthermore, we have recently shown that the polyanion-binding site is conserved in all mammalian species examined and even on avian CI)4 [23], suggesting an important functional role for this site. Finally, there is a very recent report demonstrating that lymphocyte function-;issociated antigen-1 (I,FA-I), a member of the integrin family and an important cell-adhesion molecule on lymphocytes, possesses an anionic sugar-binding region [24].‘I’he binding site is predominantly localized to the a chain of LFA-I and interacts with SP such as dextran sulphate.

Role of carbohydrate recognition in lymphocyte recirculation As mentioned earlier in this article, it is believed that the lymphocyte selectin, I E C A M - 1 participates in the specific binding of lymphocytes to peripheral lymph node I IEV by recognizing an anionic sugar ligand on I IEV [ 3, 41. However. this conclusion was derived almost entirely from studies in vitro and there have been few experiments to demonstrate that the anionic carbohydrdte structures recognized by 1,BCAM-1 inhibit lymph node ~

Structure and Function of Glycosylation

entry of lymphocytes in vivo. Recently studies by us clearly demonstrated that anionic polysaccharides, such as a yeast phosphomannan, which interact with 1,ECAM-1, profoundly inhibit lymph node entry of lymphocytes [25], thus confirming the earlier experiments in vitro. Our analysis also revealed that, unlike lymph nodes, mannose recognition plays an important role in the splenic entry of lymphocytes 1251. Subsequent experiments have shown that lymphocytes probably enter the spleen by specifically adhering to cells with a dendritic morphology which line the marginal sinuses of the spleen. These splenic sinusoidal cells (SSC) express mannose-specific receptors, and blocking of the receptors with mannose-containing oligosaccharides and polysaccharides can completely inhibit adhesion of lymphocytes to SSC (S. A. Weston & C. R. Parish, unpublished work). Whether the mannose receptors on SSC directly play a role in lymphocyte adhesion or indirectly affect lymphocyte binding is being investigated. Whatever the outcome, these studies clearly show that carbohydrate recognition plays a critical role in many aspects of lymphocyte migration. In fact, we have known for several years that once lymphocytes have entered lymphoid organs their positioning pattern can be drastically modified by the injection of certain anion polysaccharides [ 26).

Conclusions This brief review has attempted to summarize our recent studies on carbohydrate recognition molecules on lymphocytes. Most of our work has centred on the detection and characterization of lymphocyte cell-surface receptors for anionic carbohydrate structures. This emphasis arose from our earlier observation that recognition of anionic carbohydrates appears to play an important role in lymphocyte adhesion, an observation that has subsequently been confirmed in other biological systems not considered here. It is now apparent that several well-characterized cell-adhesion molecules on the lymphocyte surface can bind anionic carbohydrates, namely CD2, CD4, CD 1 la/CD 18 &FAl), CD4S arid 1,ECAM-1. The challenge for the future is the characterization of the endogenous carbohydrate ligands for these molecules and determination of the role these ligands play in lymphocyte function. 1. Springer, T. A. (1000) Nature (London) 346, 425-433

2. I%randley,H. K., Swiedlcr, S. J. & Kobbins, 1’. W. (1990) Cell (Cambridge, Mass.) 63, 801-8h3 3. Yednock. T. A. & Kosen, S. I). (1089) Adv. Immunol. 44.3 13-378 4. Imai, Y., Singer, M., Fennie, C., I,asky, I,. & Kosen, S. I). (1901) J. Cell Hiol. 113. 1213-1221 Kylatt. I). H. & Snowden, J. M. (1084) 3. Parish, C. K., J. Cell Sci. 67, 145-3 58 6 . Chong, A. S.4;. & I’arish, C. K. (1985) Cell. Immunol. 92,277-289 7. I’arish, C. K.& Snowden, J. M. (1085) Cell. Inimunol 91,201-214 8. I’arish. C. Ii., Hogarth, 1’. M. & McKenzie, I. I;. C. (1988) Immunol. Cell I3iol. 66, 22 1-230 9. Tellam, K. I,. & I’arish, C. K. (1087) f h c h i m . Hiophys. Acla 930, 55-64 10. Hradbury, M. G.& Parish, C. K. ( I O O I ) Immunology 72,231-238 11. Thomas, M. I,. (1080) Annu. Rev. Immunol. 7, 33 9- 300 12. Meuer, S. C., Hussey. K. t

Carbohydrate recognition molecules on lymphocytes.

Sti uctiire m d Function of Clycosyl~itiori Biochemical Society Transactions 296 expression of (;A(; receptors. This study confirmed at the niolec...
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