Paradoxical effects of LPS on the T-cell-independent type 2 anti-H-2 alloantigen antibody responses to allogeneic erythrocytes.

Murine allogeneic red blood cells (RBC) induce primary IgM antibody responses to H-2 alloantigens T-cell-independently (TI). In this study we showed that bacterial lipopolysaccharide (LPS), which should activate B lymphocytes polyclonally, could not trigger an anti-H-2d plaque-forming cell response. We then demonstrated that administration of LPS (on days 0-1) with allogeneic RBC suppressed the response of mice to H-2d, while giving LPS 4-6 days after RBC augmented the response. In contrast, LPS did not enable allogeneic spleen cells to induce an anti-H-2d response. Additional experiments showed that the allogeneic RBC behave as a TI class 2 antigen. It was concluded from these results that allogeneic RBC display a peculiar activity that exclusively triggers a TI type-2 B cell response that cannot be initiated by LPS and is modulated by LPS in an abnormal fashion. The possible significance of this finding in the mechanism of occurrence of natural H-2-specific IgM alloantibodies in aged mice is discussed.