Progressive disease or protective immunity to Leishmania major infection: the result of a network of stimulatory and inhibitory interactions.

The study of experimental infection of inbred strains of mice with the intracellular protozoan parasite Leishmania major has contributed significantly not only to our understanding of this fascinating host/parasite relationship but also to that of many basic immunological phenomena. Much has been learned about the cognate interaction of antigen-specific T cells and antigen-presenting cells, about cytokine and T cell subset regulation, and the requirements for costimulation. Specifically, the immune response to experimental L. major infection is the paradigm for polarized T helper cell (Th) 1 and Th2 differentiation. In this model system a Th1 response characterized by interleukin (IL)-2 and interferon (IFN)-gamma secretion leads to self-curing disease, whereas a Th2 response (IL-4, IL-10) leads to nonhealing disease. Numerous manipulations, including the injection of cytokines and of neutralizing anti-cytokine antibodies, cytokine transgene expression, and more recently cytokine and cytokine receptor gene knockout studies, have all provided intriguing new pieces to the still incomplete mosaic of our understanding of the immune response. Some of these findings were clearly unexpected and are still incompletely understood. For instance, based on earlier neutralizing anti-IL-4 monoclonal antibody injection studies, IL-4 gene-disrupted BALB/c mice were expected to be unable to mount the biased Th2 response typical of the IL-4+/+ wild-type mice and to be able to control their lesions; quite unexpectedly, the BALB/c IL-4 knockout mice remain unable to heal their L. major infection. Based on these unexpected findings, we reexamine the literature in an attempt to resolve this apparent paradox and to relate the large body of experimental findings in the mouse system to that which is known about natural and experimental infections in the human.