CD4 and HIV infectious disease: Microsphere immunoassay also detected significant increases in soluble, circulating Fas in FIV-infected cats treated with both CsA and prednisolone in Phase I. Increased levels of apoptosis in CD4+ and CD19+ lymphocytes have been well-described during HIV infection, and have been correlated with viral RNA levels in plasma and a reduction in these important lymphocyte subsets [69,70].