Th1-like Antifilarial Immune Responses Predominate in Antigen-negative Persons

To characterize immune responses associated with the putatively immune state in bancroftian filariasis (that is, both microfilaria and antigen free), humoral and cellular responses were compared among antigen- and microfilaria-negative, antigen-positive and microfilaria-negative, and microfilaria-positive individuals. Antifilarial isotype levels were measured by enzyme-linked immunosorbent assay. Peripheral blood mononuclear cell responses were measured by proliferation, by bioassay for interleukin-2 (IL-2) and IL-10, and by reverse transcription-PCR for IL-4, IL-5, and gamma interferon. The absence of circulating filarial antigen was associated with Th1-like responses, including significantly higher proliferative (P < 0.001) and IL-2 (P = 0.008) responses and a higher prevalence of gamma interferon (0.02 < P < 0.1) responses. Significantly elevated antifilarial immunoglobulin G4 (IgG4) levels (P = 0.0035) were associated with antigenemia, whereas microfilaremia was associated with significantly decreased antifilarial IgG2 levels (P = 0.0014). IL-4 mRNA levels were not significantly different among the three groups; however, there was a subpopulation of microfilaremic individuals who did not make detectable levels of IL-4 mRNA and who produced low antifilarial IgG4 levels compared with those of individuals who had detectable levels of IL-4 mRNA. IL-5 mRNA levels also were not significantly different among groups; however, more microfilaremic individuals produced IL-5 mRNA in response to adult filarial antigens, and total parasite-specific IL-4 and IL-5 mRNA levels were significantly correlated (P = 0.05). Although longitudinal data are not currently available, the elevated Th1-like responses in antigen- and microfilaria-negative individuals are consistent with the hypothesis that these responses contribute to protection in putatively immune individuals.