Figure 5

Modeling the selective advantage of strict CD4 dependence vs. promiscuous entry. The number of CD4 dependent (shaded) : CD4 independent (unshaded) virions after 3 rounds of replication is 27:3. This illustrative hypothetical example makes the following simplifying assumptions: CD4(−) and CD4+ potential host cells in a sufficiently activated state, and bearing the appropriate CKRs, are abundantly present in a ratio of 2 : 1, yielding an encounter with CD4+ targets once in every three virus-cell interactions, on average (transient encounters of CD4 dependent virus with CD4(−) cells are shown only for the first round); the relative level of infectious virus released per infected CD4+ cell is 3x that of an infected CD4(−) cell, and is the same for either strain of virus; the three types of permissive encounters between viruses and host cells have equal efficiencies of entry.