Receptors of effector T lymphocytes of congeneic strains of mice do not recognize public H-2 specificities and react to private H-2 specificities only. This has been established with the use of three tests: direct cytotoxicity assay of immune lymphocytes upon target cells, specific absorption of the lymphocytes on the target cells, and rejection of skin grafts at an accelerated fashion. Immunization with two private H-2 specificities in the system C57BL/10ScSn leads to B10.D2 induces formation of two corresponding populations of effector lymphocytes in unequal proportion: a greater part of them is directed against the private specificity H-2.33 (Kb), while the smaller part is towards H-2.2 (Db) private specificity. These two populations of effector lymphocytes do not overlap, as demonstrated by experiments on their cross-absorption on B10.D2 (R107), B10.D2 (R101), B10.A(2R), and B10.A(5R) target cells, as well as on mixtures of R107 and R101 targets. Following removal of lymphocytes reacting with one of the private H-2 specificities, lymphocytes specific to the other specificity are fully maintained. A mixture of target cells, each bearing one of the two immunizing private specificities, absorbs 100% of the immune lymphocytes and is totally destroyed by them. It is suggested that H-2 antigens are natural complexes of hapten-carrier type, in which the role of hapten is played by public H-2 specifities and that of the carrier determinant by either private H-2 specificities or structures closely linked to them. Various models of steric arrangement of MHC determinants recognized by receptors of effector T lymphocytes are discussed.

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The technique o f elution b y pronase of murine immune lymph node cells adherent to relevant allogeneic target cells has been developed for enrichment of effector T cell population. A fraction of killer cells has been obtained possessing cytotoxic activity which exceeds that of the initial immune lymphocyte population b y a factor of 6-8 as judged by a number of lymphocytes required for 50 % cytotoxic effect (CE). T h e gain in CE is fairly reproducible in different H-2 systems and under various lymphocyte-target incubation conditions. It appears to be due t o the quantitative enrichment of the population in cells bearing receptors to the particular H-2 antigens and giving rise to specific CE rather than t o an increase in cytotoxic activity per cell resulting from treatment with the target monolayer and pronase. The eluted killer cell fraction is characterized by a twofold increase in percentage of cells killed by anti-@ antibody, a 4-5-fold increase in the number of DNA-synthesizing cells and b y an increase in medium and largc lymphocyte content. A further enrichment in effector lymphocytes is reached by depletion of B cells a t the expense of their receptors t o Fc fragment of Ig. Macrophage monolayer fixed by glutaraldehyde lost its capacity t o absorb effector lymphocytes. T h e implications of the data obtained for further studies o n cell-mediated immunity are discussed. 1 I ] and the rosetteHowever, one should not disregard the possibility that in all the above-mentioned cases passive absorption of Ig on the T cell surface is revealed rather than its true (endogenic) receptors. In fact, receptors to both F c fragment of IgM Label release upon incubation with immune and normal lymphocytes EA: Erythrocytes-antibodies 281 the possibility is not excluded that such antibodies are actually produced by small B cell contaminants present in suspension. Experimental analysis [29, 301 of these and other ambiguous points in the interpretation of such results have raised doubts whether the above cited reports really concern the endogenic T receptors. .Introduction'Meanwhile, T cell receptors responsible for interaction with target cells (TC) in the H-2 system have been shown to differ from Ig of the known classes [31-331. Such receptors are manifested in the ability of cytotoxic lymphocytes t o get selectively attached for several hours at 25-37 OC t o the naturally immunosorbent-monolayer of the relevant TC ' [34-361. To study the properties o f the effector T cells, including the nature of their receptors specific to the H-2 antigens, it seems t o be essential t o isolate these cells from the whole population where they occur in a ratio of 1-4 ' %, [ 3 7 -~3 9 ] , and to concentrate them.The present work describes a technique developed for the enrichment of the effector T lymphocyte population in cells reactive to the particular H-2 antigens. T h e ability of pronase to completely and reversibly prevent immune lymphocytes from absorption on T C monolayer with no damage of the latter described earlier [40] is t...

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