Reduced numbers of CD4+ suppressor cells with subsequent expansion of CD8+ protective T cells as an explanation for the paradoxical state of enhanced resistance to Leishmania in T-cell deficient BALB/c mice.

Compared to their normal, T-cell competent counterparts, BALB/c mice that have been thymectomized, lethally irradiated, and reconstituted with bone marrow cells (TXB) were found to be resistant to Leishmania major. Even though TXB mice possess less than 30% of the normal number of T cells in their lymphoid organs, they generated a protective immune response that prevented the progressive multiplication of the parasite in the primary cutaneous lesion and its dissemination to distant visceral sites. Studies of TXB mice depleted of residual CD4+ or CD8+ T cells by systemic monoclonal antibody (mAb) treatment showed that this protective immunity depends on a residual, radio-resistant CD8+ T-cell population, and that it develops in the virtual absence of CD4+ T cells. This immune response can be negated by an infusion of CD4+ T cells from normal susceptible donors, provided that the donor cells are infused before the recipients' response is generated. It is therefore apparant that TXB BALB/c mice are more resistant than T-cell competent BALB/c mice because the former mice lack a threshold number of CD4+ suppressor T cells necessary to down-regulate the induction and expansion of CD8+ protective T cells.