Coreceptor Expression and Cellular Tropism

Take the case of primary human macrophages, for example, which express CXCR4 but are not generally productively infected with CXCR4-using isolates 

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Several recent studies have also investigated whether CD4 and chemokine receptors exist in a complex on target cells (67-70). To what extent viral entry requires formation of a competent receptor complex remains to be determined. Certainly, cellular tropism is affected by the levels of expression of CD4 and specific coreceptors, but might also depend on the activation, configuration, and modification of these coreceptors, as well as on additional cellular factors regulating steps after viral entry. 

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