BALB/c mice are highly susceptible to Leishmania major infection. They develop a progressive fatal disseminating disease even with a minimum infecting dose. However, these mice are able to contain the disease if they are exposed to sublethal γ‐irradiation shortly before infection. Earlier studies demonstrated that CD4⁺ T cells from mice which had recovered from infection (Tr) can adoptively transfer resistance. In contrast, CD4⁺ cells from mice with progressive disease (Ts) not only failed to protect, but can reverse the protective effect of the Tr cells.

Spleen cells from BALB/c mice which had recovered from L. major infection or which had progressive disease were cultured with leishmanial antigens in vitro. The culture supernatant from spleen cells of recovered mice (TrSN) contains high levels of macrophage‐activating factor (MAF) activity which can activate peritoneal macrophages to kill ⁵¹Cr‐labeled P815 cells and to eliminate intracellular parasites as measured by the reduction in [³H]thymidine uptake by residual parasites released from macrophages following sodium dodecyl sulfate treatment. The MAF activity of TrSN parallels that of recombinant interferon‐γ (IFN‐γ). In contrast, culture supernatant of spleen cells from mice with progressive disease (TsSN) contains no detectable MAF but it is able to neutralize the MAF activity of TrSN. The MAF‐inhibiting function of TsSN appears to be mediated by interleukin (IL) 3 and IL 4, since the MAF activity of TrSN and rIFN‐γ also can be inhibited by the addition of rIL 3 and rIL 4 but not by rIL 1 or rIL 2. Furthrmore, the MAF‐inhibiting activity of TsSN can be partially reversed by the addition of specific anti‐IL 3 or anti‐IL 4, but completely reversed by the combination of the two antibodies in vitro. These findings provide a mechanism for the immune regulation in leishmaniasis and a means by which the two subsets of CD4⁺ T cells influence each other through their modulation of macrophage function.