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Interleukin-4 and Transforming Growth Factor β Have Opposing Regulatory Effects on Gamma Interferon-Mediated Inhibition of Cryptosporidium parvum Reproduction

ABSTRACT It was shown previously that enterocytes activated by gamma interferon (IFN-γ) are efficient effector cells in the killing of Cryptosporidium parvum. How this function is regulated is not clearly understood, but transforming growth factor β (TGF-β) and the Th2 regulatory cytokines may play a role. Using an in vitro cell culture system, we investigated how the key regulatory cytokines interleukin-4 (IL-4), IL-10, IL-13, and TGF-β might modulate the effect of IFN-γ in inducing resistance to infection in enterocyte cell lines. The results showed that TGF-β can abolish the inhibitory effect on C. parvum development and that neither IL-13 nor IL-10 influenced the action of IFN-γ. In contrast, IL-4 cooperated with low concentrations of IFN-γ (1 and 10 U/ml) to enhance parasite killing. One mechanism that appeared to be involved in the combined activity of IFN-γ and IL-4 was intracellular Fe2+ deprivation, but induction of nitric oxide production was not involved. In one cell line, the extents and durations of phosphorylation of STAT1, a transcription factor involved in IFN-γ signaling, were similar when cells were stimulated with IFN-γ alone and with IFN-γ and IL-4γ, suggesting that the cooperative effect of the cytokines was not related to STAT1 activation. The effects of the presence of TGF-β and IL-4 on IFN-γ function did not appear to involve any alteration in the level of expression of IFN-γ receptors.