Regulation of cell-mediated immunologic reactivity to Moloney murine sarcoma virus-induced tumors. II. Nature of blocking and unblocking factors in serum.

Serum blocking factors, detected in sera of inbred CBA and BALB/c mice with progressing virus-induced tumors by their interference with leukocyte-adherence inhibition (LAI) with the use of syngeneic leukocytes, were found to be strain related: No blocking occurred with allogeneic leukocytes. Furthermore, the unblocking factors of regressor serum were also strain related: No unblocking of allogeneic progressor serum was observed. There was no requirement for H-2 compatibility between leukocytes and tumor cells from which the LAI-inducing antigen was obtained. Absorption of CBA blocking serum with anti-I-Jk antiserum (and with anti-Ik containing this antibody specificity) removed blocking activity; absorption of regressor serum did not reduce its unblocking activity. These observations indicated that certain blocking factors resembled other suppressor factors in possessing antigenic determinants coded by the I-J subregion of the major histocompatibility complex. Their strain restriction and antigen specificity suggested combination with reactive lymphocytes in blocking LAI. The properties of unblocking factors suggested they combine with blocking factors and resemble antiidiotypic antibodies.