Opportunistic infections in immunocompromised HIV-infected patients

Infection of cells of the immune system by HIV commonly results in immunodeficiency and the development of opportunistic infections. The major immune defect is an impairment of cell-mediated immune responses and, therefore, most opportunistic infections are with microorganisms which replicate intracellularly, tend to be latent, and/or are controlled by activated macrophages or cytotoxic T-cells. Thus, infections with protozoa, fungi, mycobacteria, intracellular bacteria and certain viruses, especially herpesviruses, are commonly encountered. Some of these opportunistic pathogens are unusual and several have been described for the first time through studies on patients with AIDS. In addition, some HIV-infected patients have recurrent infections by bacteria which are common pathogens in patients with antibody deficiency suggesting that other types of immune defect are sometimes present. The immune defect underlying the cellular immunodeficiency is almost certainly multifactorial, but depletion of CD4+ T-cells is the most characteristic abnormality. The blood CD4+ T-cell count is a good indicator of the severity of impaired cellular immune responses and can be used to determine when a patient is at risk of developing disease caused by an opportunistic infection. By this means, patients at risk of developing disease from reactivation of Pneumocystis, toxoplasma, mycobacterial, cryptococca] and some herpesviius infections can be identified and prophylactic anti-microbial therapy given to suppress the infections. Whilst disease caused by opportunistic pathogens is usually the result of the immune system failing to mediate a protective immune response, it is being increasingly recognised that restoration of immunity to such pathogens following suppression of HIV replication by anti-retroviral therapy may result in inflammation in tissues sub-clinically infected by them. This appears to be a particular problem with mycobacteria and herpesviruses and will become a problem of increasing importance as the effectiveness of anti-retroviral therapy improves.