How do co-receptors contribute to the process of HIV infection?

Co-receptors play a critical role in the process of HIV infection by aiding the virus in binding to and entering host cells. Specifically, HIV primarily targets CD4 T-cells, which are a type of immune cell that has a CD4 receptor on its surface. In addition to this primary receptor, HIV also requires additional co-receptors, namely CCR5 or CXCR4, to facilitate entry into the cell.

When HIV approaches a susceptible host cell, the virus first engages the CD4 receptor. This interaction is necessary but not sufficient for entry. The binding to the CD4 receptor induces conformational changes in the viral envelope proteins that allow HIV to subsequently bind to the appropriate co-receptor. Once both the CD4 receptor and co-receptor are engaged, this triggers fusion of the viral and host cell membranes, leading to the entry of the viral genetic material into the host cell.

This dual receptor engagement is essential for the virus's infectivity, as without binding to both the CD4 and co-receptor, HIV cannot effectively enter the host cell to establish an infection. Therefore, the co-receptors are pivotal in the initial stages of the HIV life cycle.