CD4 and infection: These studies found that macrophages and DCs derived from both humans and non-human primates were susceptible to infection with either a wild-type Asibi YFV or the 17D vaccine virus, that the two viruses stimulated different cytokine responses in these cells and that human DCs infected with the 17D virus were able to stimulate CD4+ T cells while those infected with Asibi virus did not.