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. Although serum reactivity to all the viral proteins is detectable, virus neutralising titres are generally low and directed against
the immunising virus strain (type specific immunity). Passive transfer of antibody from asymptomatic to symptomatic patients is claimed to be beneficial, but this requires confirmation. Antibodies to HIV may even facilitate infection of cells bearing immunoglobulin (Fc) receptors, such as monocytes. In AIDS a fall in the titre of antibodies to core protein (p24) is often associated with disease progression. p24 antigen, which is
detectable in the serum of some patients, may show a rise at the same time and has been used as a marker of disease progression.

      CD8 cytotoxic lymphocytes (CTL) capable of killing HIVinfected 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.