A partial characterization of suppressor cells in the spleens of mice conditioned with fractionated total lymphoid irradiation (TLI)

Total lymphoid irradiation (TLI) is a highly effective modality for inducing immunosuppression and transplantation tolerance. The cellular basis for this immunosuppression is not clear although T cells have been implicated. To study further the effect of TLI on the immune system, we have examined the B cells and suppressor cells in the spleens from TLI-conditioned mice. Our results indicate that after TLI, the spleen is rapidly repopulated with many large, immature cells. The probable source of these cells is the shielded bone marrow (BM). The B cells from TLI-conditioned mice are transiently immature and hyporesponsive in vitro to a T-independent antigen. Spleen cells from TLI-conditioned mice nonspecifically suppress the in vitro T-independent anti-TNP response of normal B cells. The suppressor cells lack both B and T cell markers and adhere to Sephadex G-10. The suppressor cells in spleens from TLI-treated mice bear a number of similarities to those present in normal BM. When normal BM cells were analyzed by indirect immunofluorescence for the presence of the Mac-1 antigen, two populations of suppressor cells could be identified: one was Mac-1⁺ and the other was Mac-1^-. These data are consistent with the possibility that a subpopulation of the suppressor cells found in normal BM and in the spleens from TLI-conditioned mice are immature cells of the monocytic/granulocytic lineage.