Alternative versus classical macrophage activation during experimental African trypanosomosis.

African trypanosomes are extracellular parasites causing sleeping sickness to human or nagana to livestock in sub-Saharan Africa. To gain insight into factors governing resistance/susceptibility to these parasites, the immune responses in mice infected with a Trypanosoma brucei phospholipase C null mutant (PLC(-/-)) or its wild type counterpart (WT) were compared. We found that the T. b. brucei mutant inducing a chronic infection triggers the production of type I cytokines during the early stage of infection, followed by the secretion of type II cytokines in the late/chronic phase of the disease. In contrast, WT-infected mice are killed within 5 weeks and remain locked in a type I cytokine response. The type I/type II cytokine balance may influence the development of different subsets of suppressive macrophages, i.e. classically activated macrophages (type I) versus alternatively activated macrophages (type II) that are antagonistically regulated. Therefore, the phenotype and accessory cell function of macrophages elicited during WT and PLC(-/-) T. b. brucei infections were addressed. Results indicate that classically activated macrophages develop in a type I cytokine environment in the early phase of both WT and PLC(-/-) trypanosome infections. In the late stage of infection, only PLC(-/-)-infected mice resisting the infection develop type II cytokine-associated alternative macrophages. In parallel, we found that mice susceptible to Trypanosoma congolense infection, showing an exponential parasite growth until they die, have a higher level of type II cytokines in the early stage of infection than resistant animals controlling the first peak of parasitaemia. The levels of type I cytokines were comparable in both T. congolense-resistant and -susceptible mice. On the basis of these results, we propose that survival to African trypanosome infection requires a type I cytokine environment and classical macrophage activation in the early stage of infection, enabling mice to control the first peak of parasitaemia. Thereafter, a switch to type II cytokine environment triggering alternative macrophage activation is required to enable progression of the disease into the chronic phase. The possible role of the sequential activation of alternative macrophages in the late/chronic stage of infection in the increased resistance of mice to PLC(-/-) T. b. brucei will be discussed.