Infection with Brugia microfilariae induces apoptosis of CD4+ T lymphocytes: A mechanism of immune unresponsiveness in filariasis

Abstract

In humans infected with lymphatic filariasis, microfilaraemia [the presence of microfilariae (Mf) in the blood] is generally associated with both poor antigen (Ag)-specific proliferative responses and with protection from severe disease. Clonal deletion has been suggested as one possible mechanism by which parasite-reactive lymphocytes, that may be capable of mediating resistance and/or immunopathology, are silenced in asymptomatic carriers. In this study we demonstrate that splenic lymphocytes from mice infected with microfilariae of Brugia pahangi display an Ag-specific proliferative defect. However, these cells were not completely unresponsive since they produced high levels of Ag-specific IFN-γ. Using TdT-mediated dUTP-biotin nick end labeling for flow cytometry, CD4+ lymphocytes from Mf-infected mice cultured with Ag showed high levels of apoptosis when compared to those from L3-infected mice which proliferated well in response to Ag. Treatment of Ag-stimulated cultures with aminoguanidine (AMG), an inhibitor of inducible nitric oxide synthase, rescued the CD4+ T cells from apoptosis and reversed the proliferative defect. Furthermore, carboxyfluorescein diacetate succinimidyl ester labeling allowed the visualization of dividing CD4+ T cells in cultures from Mf-infected animals only in the presence of AMG. We hypothesize that CD4+ T cells indirectly trigger their own apoptosis by secreting significant quantities of IFN-γ resulting in the induction of high levels of nitric oxide, and the subsequent elimination of effector T cells. Our findings are the first direct evidence that infection with Brugia Mf can selectively induce lymphocyte apoptosis, a phenomenon that could contribute to the proliferative defect and parasite persistence associated with the microfilaraemic state in the infected human.