An analysis of immunosuppression generated by the graft-versus-host reaction.

The kinetics and cellular characteristics of immunosuppression in (CBAxC57BL)Fl mice during graft-versus-host (GVH) reaction induced with spleen cells from either parental strain have been investigated. Plaque forming cell (PFC) responses to a thymus-dependent antigen chicken red blood cells (CRBC) and a thymus-independent antigen levan (LE) were suppressed to a comparable extent. The unresponsive state which developed in GVH mice was expressed also by their spleen cells following transfer into irradiated recipients. GVH spleen cells also suppressed the response of normal spleen cells in a mixed transfer system, but this effect was lost completely by day 21 whereas the original donors (or their cells) remained refractory after 80 days. This active suppressor effect was mediated by T cells of parental donor origin, as demonstrated by cell fractionation studies, anti-Thy 1.2 (theta) treatment and selective deletion of either donor or host cells. No evidence for macrophage participation could be found from several in vivo or in vitro experiments. Indeed, the immune response of normal cells to dinitrophenylated levan (DNP-LE) was inhibited by GVH-T cells when both populations were depleted of macrophages. Experiments in double Marbrook cultures which utilized a cell-impermeable membrane, demonstrated that suppression was mediated, at least partially, by a soluble factor. Spleen cells from (CBAxC57BL)Fl mice undergoing GVH reaction induced by C57BL cells were depleted of their FI component with anti-CBA alloantiserum. The suppressive activity of the residual donor cells was still expressed against other FI cells (AKR x C57BL) which were H-2 compatible with the original host, but not against H-2 incompatible cells (DBA/lxC57BL)Fl. However, the latter were suppressed in the additional presence of (CBAxC57BL)Fl cells. Thus, interaction of donor T cells with FI target cells containing H-2 antigens towards which they are sensitized is mandatory for the subsequent manifestation of immunosuppressive activity. Suppressor T cells were characterized with anti-Ly and anti-la[k] sera and found to be Ly 1[+]2[+]3[+]la[+]. From experiments in which B cells from GVH mice were supplemented with purified normal T cells, it is concluded that 1) the target for suppressor cell activity is a B cell, although macrophages cannot be excluded and 2) suppression is mediated by a reversible antiproliferative effect. One or two injections of azathioprine (approximately 200 mg/Kg) into FI recipients 2 to 3 days after the induction of lethal GVH disease with C57BL parental cells abrogated the subsequent mortality. However, a delayed onset of immunosuppression was observed in some animals, indicating that a residual 'low-grade' GVH reaction persisted. This outcome was mimicked by inducing GVH reactions with low numbers of C57BL parental cells. Surprisingly, azathioprine was ineffective in preventing mortality in 600r-irradiated FI recipients which were undergoing GVH reaction. The delayed onset of suppression in GVH-mice treated with azathioprine was present when GVH reactions were induced with parental lymph node, but not with bone marrow cells. Since GVH-induced suppressor T cells were not sensitive to treatment with azathioprine in vitro, it is considered that this drug exerts an anti-proliferative effect on parental GVH-reactive T cells