T helper responsiveness in human Loa loa infection; Defective specific proliferation and cytokine production by CD4(+) T cells from microfilaraemic subjects compared with amicrofilaraemics

The proliferation and cytokine profiles of peripheral blood mononuclear cells (PBMC) from microfilaraemic (Mf(+)) subjects infected by Loa loa in response to antigens of several parasitic stages were compared with those from amicrofilaraemic (Mf(-)) individuals. While a strong lymphoproliferative response and consistent levels of both Th1 (IL-2, interferon-gamma (IFN-gamma)) and Th2 (IL-4, IL-5) type cytokines were observed in response to adult worm (AW) and microfilariae (Mf) antigen in Mf(-) individuals, Mf(+) subjects were characterized by a T cell unresponsiveness, including proliferation, cytokine production and IL-2 mRNA expression. Conversely, T cell responsiveness to mitogens and non-specific antigen were similar in the two endemic populations. Depletion of lymphocyte subpopulations indicated that T CD4(+) were mainly involved in the specific cellular response. In contrast to other cytokines, IL-10 was produced in response to all parasitic stages, in both Mf(+) and Mf(-) patients. Neutralization of IL-10 did not restore cytokine production in Mf(+) patients, while B7 mRNA expression was similar between Mf(+) and Mf(-) subjects in response to Mf antigen, suggesting that IL-10 was not the only factor responsible for T cell unresponsiveness. Mf(+) patients have lower Mf antigen-specific IgG levels compared with Mf(-), and there is a significant correlation between Mf antigen-specific antibodies and IL-5 responses. These findings suggest that Mf(-) status is correlated with T helper responsiveness, including proliferation and production of both Th1- and Th2-type cytokines, whereas Mf(+) status is characterized by unresponsiveness of the same cell population, induced and/or maintained by microfilariae.