We have examined the temporal sequence of events leading to the formation of hepatic granulomas after the intravenous injection of L. donovani amastigotes into BALB/c mice. Parasite ingestion by permissive Kupffer cells (KC) occurred promptly, and local KC aggregations were the foci about which granulomas were subsequently formed. Infected KC were recognized by the uptake of colloidal carbon and the expression of the macrophage-specific antigen recognized by F4/80 mAb. Peroxidase-positive granulocytes migrated rapidly and were followed by monocytes and L3T4+ T cells that enclosed the infected KC. Thereafter, Ly-2+ T cells were prominent members of the granulomatous lymphoid population. Parasites multiplied until 4 wk, and then a prompt reduction in infected cells occurred. This was associated with a sharp decline in the L3T4+ T cells of the granulomas and the maintenance of the Ly-2+ subset. In comparison, athymic nu/nu mice developed smaller, more slowly appearing granulomas that contained granulocytes and monocytes and exhibited progressive parasite replication. Upon rechallenge, the entire process was completed in 2 wk, and infected KC in the euthymic mice were never observed. We hypothesize that the effectiveness of the granulomatous response requires the destruction of parasitized host cells (KC), in a lymphokine rich environment. We further suggest that the Ly-2+ T cell serves as an important effector cell in this process, either by direct cytotoxicity or by supporting the cytotoxic potential of other cell types in the granuloma.
Local cellular responses to cutaneous infection with Leishmania mexicana amazonensis were examined in susceptible (BALB/c) and resistant (C57BL/6) mouse strains by immunocytochemical and electron microscopic studies. Infection during the first 8 wk in both animal strains was characterized by progressively enlarging lesions, epidermal thickening and ulceration, and accumulation of eosinophils and Ia+ infected macrophages. Healing of C57BL/6 mouse lesions began after 12 wk of infection and was associated with local influx of both Th (L3T4+) and T cytotoxic/suppressor (Lyt-2+) cells into the dermis, and Ia antigen expression on epidermal keratinocytes. T lymphocyte infiltration was marked and intracellular parasites were scarce by 21 wk of C57BL/6 infection. Similarly, granulomas in C57BL/6 livers contained L3T4+ and Lyt-2+ T lymphocytes and no visible intracellular parasites by 21 wk of infection. In contrast, BALB/c mouse lesions continued to enlarge and never healed. Throughout the entire course of infection, T lymphocyte influx into the heavily infected dermis was minimal. Keratinocyte Ia expression was absent in BALB/c lesions. BALB/c livers were heavily infected by 18 wk of cutaneous infection, with few demonstrable T lymphocytes. A systemic absence of T cells could not be demonstrated in BALB/c mice. Both L3T4+ and Lyt-2+ T cells were found in the peripheral blood in normal numbers in both mouse strains. Our results support the role of T cells as important local effector cells in the healing response of murine cutaneous leishmaniasis. We suggest that local T lymphocyte infiltration may provide lymphokines, particularly IFN-gamma, that can activate infected macrophages to destroy the intracellular parasites. Alternatively, T cells may play a cytotoxic role, killing infected macrophages and allowing local humoral factors to destroy released extracellular parasites.