Loading of peptide on to MHC molecule is not essential for clearance of Cryptosporidium parvum.

Transgenic and knockout mice usefully model the mechanisms that result in the clearance of Cryptosporidium parvum from the gut. CD4+ cells, cells expressing MHC class II, and CD154/CD40 interactions are essential. Unexpectedly, AND RAG-/- and DO11.10 RAG-/- mice with single specificities of T cells successfully clear Cryptosporidium infection. Clearance is accompanied by activation of CD4+ cells in the MLN. The ability of T cells bearing receptors for apparently irrelevant and non-cross reactive antigens to activate and to clear infection is surprising. The requirement for class II MHC expression for Cryptosporidium clearance raises the alternative possibilities that (a) class II MHC is required to present a peptide that is loaded as a consequence of infection or (b) that the cytokine environment engendered by a Cryptosporidium infection allows affinity for self MHC to activate naive T cells. In order to test the hypothesis that peptide loading is necessary, we used A betaE alpha-/-Ii-/- mice that express a hybrid IA-IE MHC molecule. They also carry a transgene that makes an E alpha peptide while disruption of their invariant chain blocks the loading of a foreign peptide on to their MHC class II molecules. After oral gavage, the course of infection was followed by ELISA. CD4+ cells in the MLN of these mice were activated to express CD69 and the infection was cleared. We conclude that the loading of a Cryptosporidium or other infection-dependent peptide onto the MHC class II molecules of APCs is not necessary for clearance of Cryptosporidium. Instead the TcR affinity for self-MHC must suffice for T cell activation in the cytokine environment resulting from infection.