Koch's postulates and the molecular ,biology of T-cell function

How can the function of a molecule on the surface of a T lymphocyte be determined? The advent of monoclonal antibodies (mAbs) and clonal populations of T cells seemed to make this task relatively simple: blocking of a particular function of a cloned T cell by a well- characterized mAb seems to provide ~easonable evi- dence for the involvement of the structure recognized by the mAb with the function that is blocked. For structures with demonstrable receptor function, adaptation of 'classical' approaches of endocrinology and cellular physiology assessing receptor-ligand interactions can provide confirmatory information. If a pure, reco,'qbinant lymphokine binds to a cell with characteristics appropri- ate for a hormone receptor (saturable binding with a reasonably high affinity, and ligand and target cell specificity), and if this binding is blocked by a mAb, it seems logical to designate the structure recognized by the antibody as the receptor for that lymphokine. However, defining functions of cell surface molecules by the inhibitory properties of mAbs is less satisfactory for complex structures and those for which a putative ligand has not been isolated in a form suitable for binding studies. The T-cell receptor (TCR) for antigen and several 'accessory' molecules found on T lymphocytes fit in this category. CD4 and CD8 are members of the immunoglobulin gene superfamily which are expressed on mutually exclusive subsets of mature peripheral T lymphocytes. The presence of these molecules on mature T cells generally correlates better with the class of major histocompatibility complex (MHC) antigen which restricts the reactivity of the cell than with its functional charac- teristics: CD4 is found on human and murine T cells which react with, or whose reactivity is restricted by, class II MHC antigens, while CD8 is found on human and murine T cells which react with, or whose reactivity is restricted by, class ! MHC antigens ~-3. This correlation, and the fact that mAbs reactive with CD4 or CD8 inhibited antigen-specific functions of T cells bearing these structures, suggested that CD4 and CD8 react with monomorphic determinants expressed on class II and I MHC molecules, respectively. However T cells vary in susceptibility to inhibition by anti-CD4 or anti-CD8 mAbs. For example, the cytotox- icity of most cloned cytolytic T lymphocytes (CTL) derived from primary mixed leukocyte cultures (MLC)is inhibited by anti-CD8 mAb, but this is not true for most cloned CTL derived from MLC prepared with spleen cells from mice sensitized in vivo with alloantigens 4.s. Cloned CI"L less susceptible to inhibition by anti-CD8 mAb could also lyse target cells expressing low levels of specific allo- antigen, whereas cloned CTL highly susceptible to inhibi- tion by anti-CD8 mAb could not 6. Also, using a cloned CTL whicl~ interacted with two different target cells through the same TCR but with different avidities (based on cold target inhibition studies), the apparently lower The Ben May Institute, the Department of Pathology, and the Commit- tee on Immunology, The University of Chicago, Chicago, IL 60637, USA,