CD4 and viral infectious disease: In vitro, cell culture experimental systems, mostly using macrophages derived from the differentiation of purified blood monocytes (MDMs) or the human promonocytic CD4+ THP-1 cell-line, definitively showed that these myeloid cells are difficult to infect with HIV-1 and are less susceptible to cell-free virus infection compared to activated primary CD4+ T cells [85,86,87].