Nonencapsulated Trichinella pseudospiralis Infection Impairs Follicular Helper T Cell Differentiation with Subclass-Selective Decreases in Antibody Responses.

Infectious microorganisms often modify host immunity to escape from immune elimination. Trichinella is a unique nematode of the helminth family, whose members parasitize the muscle cells inside the host without robust eliminative reactions. There are several species of Trichinella; some develop in muscle cells that become encapsulated (e.g., Trichinella spiralis) and others in cells that do not encapsulate (e.g., Trichinella pseudospiralis). It has already been established that Trichinella infection affects host immune responses in several experimental immune diseases in animal models; however, most of those studies were done using T. spiralis infection. As host immune responses to T. spiralis and T. pseudospiralis infections have been reported to be different, it is necessary to clarify how T. pseudospiralis infection influences the host immune responses. In this study, we investigated the influence on host humoral immunity in T. pseudospiralis-infected mice. We demonstrated that T. pseudospiralis infection decreased antigen-specific IgG2a and IgG2b antibody (Ab) production in mice immunized with a model antigen. This selective decrease in gamma interferon (IFN-γ)-dependent Ab production was not due to a decrease in IFN-γ production, and we instead found impaired follicular helper T (Tfh) cell differentiation. The affinity maturation of antigen-specific Ab tended to be delayed but was not significant in T. pseudospiralis-infected mice. We also observed that CD11b(+) spleen cells in T. pseudospiralis-infected mice expressed CD206 and PD-L2, the phenotype of which was M2 macrophages with weak production of interleukin-6 (IL-6), possibly resulting in impaired Tfh differentiation. Taken together, our results indicate that nonencapsulated Trichinella infection induces selective dampening in humoral immunity with the suppression of Tfh differentiation.