Pathogenesis of infections in HIV-infected individuals: insights from primary immunodeficiencies

Abstract

Following infection with almost any given microorganism other than an emerging pathogen, only a minority of individuals develop life-threatening clinical disease, implying that these individuals have some form of immunodeficiency. A growing number of inherited and acquired immunodeficiencies have been deciphered over the last 50 years. HIV infection is probably the best-known acquired immunodeficiency. It emerged about 40 years ago and precipitates various severe infections, the occurrence of which is associated with a fall in circulating CD4⁺ T cells. However, despite the strength of this correlation, infection rates differ between patients with similar levels and durations of CD4⁺ T lymphopenia in the presence or absence of antiretroviral treatment. Moreover, a few infections seem to be less dependent on total CD4⁺ T-cell levels. The fine detail of the mechanisms underlying these infections is unknown. We discuss here how studies of the human genetics and immunology of some of these infections in patients with primary immunodeficiencies (PIDs) have provided unique insights into their molecular and cellular basis. Defects of specific CD4⁺ Th-cell subsets account for some of these infections, as best exemplified by Th1* for mycobacteriosis and Th17 for candidiasis. PIDs are individually rare, but collectively much more common than initially thought, with new disorders being discovered at an ever-increasing pace and a global prevalence worldwide approaching that of HIV infection. Studies of known and new PIDs should make it possible to dissect the pathogenesis of most human infections at an unprecedented level of molecular and cellular precision. The predictive, preventive, and therapeutic implications of studies of immunity to infection in PIDs may extend to HIV-infected patients and patients with infectious diseases in other settings.

Figure 1

PID gene defects in the Th17 pathway. Fungal infections recognized by phagocytes and epithelial cells trigger the secretion of cytokines, which induce Th17 cells and Th17 production. Genetic defects of this pathway have been identified for IL12B, IL12RB1, STAT3, STAT1 (GOF), RORC, IL17F, IL17RA, IL17RC, ACT1. Autoantibodies made in APECED target IL-17A, IL-17F and IL-21, thereby blocking the pathway.

Figure 2

PID gene defects in the IFN-γ pathway. Intracellular mycobacterial infection activates the loop of the IFN-γ pathway connecting phagocytes and T cells. In addition to its crucial functions in TCR signaling, the NF-??B pathway also enhances IFN-γ responses by inducing the secretion of IL-12. Genetic defects of this pathway have been identified for ISG15, IL12B, IL12RB1, TYK2, IFNGR1, IFNGR2, JAK1, STAT1, IRF8, GATA2, CD40L, NEMO, IKBKB, NFKBIA, and CYBB.