Specific immune responses to HIV

In spite of the fact that HIV-infected individuals show the gross abnormalities of immune function described above, they are able to mount a specific immune response to HIV itself.

CD8 cytotoxic lymphocytes (CTL) capable of killing HIV- infected targets are detected in most HIV-infected individuals and may be beneficial. This is suggested by the observation that viraemia declines at the time that CTL are first detected following infection, and in patients with stable disease, a high frequency of CTL is detectable in the peripheral blood. In addition, in individuals who have been regularly exposed to HIV while remaining seronegative and without detectable virus, HIV-specific CTL have been detected. As well as killing infected cells directly, CD8 lymphocytes may contribute to protection by producing several chemokines and CAF (CD8 T-cell antiviral factor), which strongly inhibit viral replication in CD4 cells. All this has led to the suggestion that CTL are an effective protective mechanism. However, because reverse transcription is an error-prone process, virus mutants arise, which evade the CTL response (escape mutants). These mutants may not only evade recognition themselves but also inhibit recognition of un­mutated virus.

There is some evidence to suggest that a minority of patients mount a specific CD4 T-cell response to HIV and that this is associated with effective control of virus replication. In animal experiments CD4 cells have been shown to be important for the maintenance of an effective CTL response, which may explain this association.