Distinct function of Th1 and Th2 type delayed type hypersensitivity: protective and pathological reactions to chlamydial infection.

The role of delayed-type hypersensitivity (DTH) to chlamydial infection has been shown to be a double-edged sword to the host. Reported animal and human studies have, on the one hand, shown that DTH is associated with protective immunity against chlamydial infection and, on the other hand, shown links to immunopathology. Using a murine lung infection model, we recently demonstrated that there might be two different functional types of DTH induced by chlamydial infection based on its association with cytokine patterns. Th1 type DTH is associated with protection while Th2 type DTH is associated with immunopathology. The Th2 type DTH demonstrated in IFNgamma gene knockout (KO) mice is characterized by eosinophil infiltration in addition to mononuclear cell infiltration that exists in Th1 DTH, observed in wild-type C57BL/6 mice and IL-10 KO mice. In addition, the inflammatory cells in IFNgamma KO mice fail to target the cellular sites of chlamydial inclusions in infected tissues and fail to clear the infection. The functional differences in Th1 and Th2 type DTH responses may account for the dual role DTH plays in chlamydial protective immunity and immunopathology.