FIG. 1.

The early effects of HIV-1 infection on the immune system pose atypical demands on a vaccine. On the one hand, the inflammatory response benefits the virus by providing an expanded pool of susceptible cells (e.g., lymphocyte activation leads to increased CCR5 expression). Conversely, CD4⁺ T lymphocytes are rapidly destroyed in the gut-associated lymphoid tissue (GALT), which deprives the B cell response of much early T cell help. Studies of the timing of postexposure passive immunization after SHIV challenge of macaques show that to prevent infection, the Ab must be given earlier than 24 h after the virus. The time window for immune prevention of the establishment of infection may be similar, and hence too short for an anamnestic response to be effective; the most feasible safety net would be preexisting high titers of neutralizing antibodies.