Allosuppression of B cells in vitro by graft‐vs.‐host reaction‐derived T cells is caused by cytotoxic T lymphocytes

Abstract

An acute graft‐vs.‐host reaction (GVHR) was induced by i.v. injection of 10⁸ lymphoid cells from C57BL/10 (B10) donors (H‐2^b/b) into adult non‐irradiated (B10 x DBA/2)F₁ mice (H‐2^b/d). Previous experiments have established that spleen cells obtained from such GVHF₁ mice suppress the primary antibody response of normal F₁ spleen cells to sheep red blood cells. This type of suppression was termed “allosuppression”, and it was shown to be mediated by Ly‐2⁺ T cells of donor origin that react against H‐2 antigens of the host. It was unclear, however, whether the actual mechanism of allosuppression was due to suppressive factors generated by donor T cells or whether the latter killed the F₁ B cells. Here, we show that for their suppressive effect GVHF₁ spleen cells need direct cell contact with F₁ spleen cells; no suppressive effect was measured in a double‐chamber culture system in which the GVHF₁ spleen cells were separated from the responding normal F₁ spleen cells by a cell‐tight membrane filter. The suppressive effect only affected cells expressing the appropriate H‐2 class I alloantigen; in mixed cultures of irradiated F₁ spleen cells and GVHF₁ spleen cells with third‐party B cells the antibody response of the third‐party B cells was not suppressed. GVHF₁ spleen cells showed cytotoxic T lymphocyte (CTL) activity specific for the allogeneic F₁ H‐2 antigen. The suppressive effect of the GVHF₁ spleen cells did not differ from that exerted by cloned CTL specific for MHC class I alloantigens; cloned CTL suppressed the primary antibody response of murine spleen cells without affecting the response of third‐party B cells added to the cultures. The combined findings show that “allosuppression” in vitro is not due to suppression of F₁ B cells, but to a direct killing of these cells by alloreactive CTL.