Masking of veto function in vivo by activated CD4+ T lymphocytes.

Donor CD8+ T lymphocytes injected into recipient mice incompatible at major histocompatibility complex (MHC) class I genes induce donor-specific CTL nonresponsiveness, attributed to the veto function of donor cells. Here we show that conditions leading to strong activation of CD4+ T cells, namely the presence in the recipient of foreign MHC class II determinants, lead to the apparent loss of veto function of donor cells. This "masking" of veto function is dependent on the dose of foreign MHC class II present. Veto function can be partially restored by treatment of recipients in vivo with CD4-specific antibody, a measure which has been shown to eliminate the function of CD4+ T cells in vivo. We conclude that CD4+ T cells activated by contact with antigen can interfere with the veto function of CD8+ T cells. Consequences of this finding are: (a) veto function of a sample cell population can be overlooked when activation of CD4+ T cells occurs simultaneously. (b) The balance between veto function of recipient cells and its abrogation might be responsible for the kind of graft-vs.-host reaction generated (CD8+ T cell-mediated and frequently lethal or CD4+ T cell-mediated and not lethal) when parental T cells are injected into recipients incompatible at MHC class I and class II genes. (c) A possible contribution of veto cells should be considered in several protocols in which donor hemopoetic cells were used in conjunction with CD4-specific antibodies to induce transplantation tolerance. (d) Veto function in vivo does not require a contribution of CD4+ T cells.