Corneal inflammation similar to human onchocercal keratitis can be induced in mice by subcutaneous immunization of a soluble extract of Onchocerca volvulus (OvAg) followed by direct injection of OvAg into the corneal stroma. Previous studies have shown that corneal pathology is associated with increased systemic and corneal Th2 cytokine expression and that IL-4 gene knockout (IL-4-/-) mice develop less severe or no O. volvulus-mediated keratitis. The current study examined the contribution of Th2 cytokines to the diminished OvAg-induced corneal immunopathology observed in IL-4-/- mice. IL-4-/- mice (129Sv x C57B1/6), wild-type F2 littermates (IL-4+/+), and C57B1/6 mice were sensitized by repeated subcutaneous immunization with OvAg. Ten days after the final immunization, mice were sacrificed, spleens were removed, and cells were incubated with OvAg. Cells from immunocompetent C57B1/6 and IL-4+/+ mice produced IL-4 and IL-5, but no IFN-gamma, whereas cells from IL-4-/- mice had elevated IFN-gamma and no IL-4. Interestingly, cells from these animals produced levels of IL-5 protein equivalent to those of C57B1/6 and IL-4+/+ mice. To determine cytokine production in corneas during the onset of onchocercal keratitis, OvAg-immunized mice were injected intracorneally with OvAg, and cytokine gene expression in the cornea was determined by RT-PCR. Temporal analysis of cytokine gene expression in corneas of immunocompetent mice showed that the Th2-associated cytokines IL-4, IL-5, IL-10, and IL-13 were produced within 1 day of intrastromal injection, with sustained elevations for 10 days. Maximal IFN-gamma mRNA levels were not detected until Day 10. This was in contrast to IL-4-/- mice in which IFN-gamma appeared at Day 1 and remained elevated for at least 10 days. Moreover, in corneas from IL-4-/- mice, all Th2 cytokines with the exception of IL-4 were up-regulated and expressed with kinetics similar to that of IL-4+/+ littermates. Histologically, corneas from IL-4-/- mice were less edematous and contained fewer eosinophils and other inflammatory cells than those from immunocompetent controls. As there was no difference in peripheral eosinophil levels, these data indicate that the diminished severity of onchocercal keratitis in IL-4-/- mice is not due to failure to develop systemic or local Th2 cytokine responses or to produce eosinophils, but that IL-4 may be involved in recruitment of eosinophils and other inflammatory cells into the corneal stroma.