STRUCTURAL RELATIONSHIPS BETWEEN INDIVIDUAL H-2 SPECIFICITIES ON THE CELL SURFACE

Stepwise incubation with specific H-2 antibody and rabbit-anti-mouse Ig was used to modulate the expression of selected H-2 specificities on the surface of lymph node cells or EL 4 leukaemia cells. Modulation was reflected in a loss of sensitivity to complement-dependent lysis and a polar pattern of immunofluorescence (‘capping’). Specificity H-2.5 on H-2^a cells was co-modulated with H-2.11 and so was H-2.11 with H-2.5 on H-2^k cells. However, when H-2.11 on H-2^k cells was modulated, the cells did not loose cytotoxic sensitivity for H-2.5. This asymmetry in co-modulation of H-2.5 and H-2.11 may be accommodated by the duplication model of the H-2 complex which postulates a double determination of H-2.5 in the H-2^k (unlike H-2^a) chromosome, with one determinant being in the K region (where also the H-2.11 determinant is located) and the other in H-2D. The modulation results with H-2.5 and H-2.11 are compatible with the view that various H-2K specificities may represent distinct antigenic sites on the same glycoprotein molecule whereas H-2D specificities are on separate molecule(s). The independent modulation of H-2D.4 and H-2K.11 specificities further indicates that molecules of the two classes are also secondarily not linked in the membrane structure. Modulation-induced cytotoxicity resistance was further shown to persist even when the specific H-2 antibody is newly added; this suggests that it is a change in the distribution of the H-2 antigen to be held responsible (rather than the loss of the sensitizing antibody). A modulated expression of a certain H-2 specificity might then be expected to affect its blocking relationship to another H-2 specificity; the blocking index expresses the degree of interference by antibody attached to one of them with the attachment of antibody to the other. The blocking relationship between H-2.4 and H-2.11 was shown to be completely abolished by the independent modulation of H-2.11. In contrast to this, the blocking index for H-2.5 and H-2.11 remained practically constant following the modulation of either specificity; this indicates that the intramolecular configuration of such two H-2 sites may not be altered by the gross redistribution of the whole molecule in the plane of the membrane. The rigidity of this configuration seems to be maintained by neuraminidase-sensitive groups as suggested by the reduced blocking index in neuraminidase pretreated cell.