Helper-dependent vs. helper-independent CTL responses in HIV infection: implications for drug therapy and resistance.

Clinical data from HIV-infected patients, as well as theoretical studies, suggest that CTL responses in the presence and absence of CD4 cell help are qualitatively different. In the presence of help, CTL responses are maintained despite very low antigenic loads and control the infection in the long term. In the absence of specific helper cell responses, CTL require high antigenic loads to be maintained, are short lived at low levels of antigen, and do not control the infection in the long term. This paper describes mathematical models analysing the dynamics of helper-dependent and helper-independent CTL in HIV infection with special focus on the dynamics during drug therapy in chronic infection. Theory suggests that a fast rate of virus spread results in high degrees of helper cell impairment which promotes the development of helper-independent CTL responses and compromised immunological control. In agreement with clinical findings, the model suggests that upon start of therapy, there is a transient increase in the level of CTL, followed by a decline to low levels once virus load has been significantly suppressed. According to the model, the presence of helper-independent CTL can promote the establishment of a helper-dependent memory response. Interestingly, this gives rise to the prediction that a relatively early stop of therapy, before the level of CTL has fallen below a threshold, can promote improved immunological control. Issues concerning the timing and duration of treatment are discussed. The CTL kinetics during drug therapy also provide new insights into the principles underlying the emergence of drug-resistant strains during the course of treatment.