CD4 and infection: Of interest B6.RAG-/- mice, which are entirely deficient of T cells and B cells, were highly sensitive to infection with an equivalent dose of HKx31 such that all mice succumbed to infection 5 days after inoculation (data not shown) arguing that other aspects of adaptive immunity such as CD4+ T cells and/or antibody responses play a critical role in control and clearance of virus from B6.TAP-/- mice.