The T-cell response is central in the adaptive immune-mediated elimination of pathogen-infected and/or cancer cells. This activated T-cell response can inflict an overwhelming degree of damage to the targeted cells, which in most instances leads to the control and elimination of foreign invaders. However, in conditions of chronic infection, persistent exposure of T cells to high levels of antigen results in a severe T-cell dysfunctional state called exhaustion. T-cell exhaustion leads to a suboptimal immunemediated control of multiple viral infections including the human immunodeficiency virus (HIV). In this review, we will discuss the role of T-cell exhaustion in HIV disease progression, the long-term defect of T-cell function even in aviremic patients on antiretroviral therapy (ART), the role of exhaustion-specific markers in maintaining a reservoir of latently infected cells, and exploiting these markers in HIV cure strategies. K E Y W O R D S chronic viral infection, HIV, immune checkpoint inhibitors, PD-1, T-cell exhaustionThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

The clinical presentation of 60 consecutive Pneumocystis carinii pneumonias in 58 HIV-infected patients (48 men, 10 women, mean age 34 [22-53] years) was prospectively evaluated from April to August 1989 and compared with 60 consecutive P. carinii pneumonias in 59 HIV-infected patients (55 men, 4 women, mean age 37.5 [22-60] years) between 1981-88. Mortality rates within 14 days after diagnosis of P. carinii pneumonia were 50% (8 of 16 patients) until 1985, 20.5% (9 of 44) between 1986 and August 1988, and 1.7% (one of 60) in 1989. The degree of severity of the pneumonias at time of diagnosis was markedly lower in 1989, as shown by following parameters (averages of 1989, compared with averages of 1981-88): lactate dehydrogenase 540 (250-1419) U/l versus 680 (235-1920) U/l (not significant); alveolo-arterial difference of partial oxygen tension (pA-aO2) 22.9 (0.5-73.5) mmHg versus 39.7 (19-70) mmHg (P less than 0.001); score of radiological findings 1.4 (0-3) versus 2.7 (0-4) (P less than 0.001). In 1989, mainly clinical symptoms (dry cough: 57 of 60 cases, dyspnea: 44 of 60 cases, fever: 43 of 60 cases) initiated the diagnostic procedure: chest radiographs, lactate dehydrogenase and pA-aO2 were normal in 13, 25 and 33 episodes, respectively. The lower mortality rate of P. carinii pneumonia could not primarily be explained by therapeutical progress since the treatment of choice did not change fundamentally since 1981. Above all, early diagnosis fundamentally determined the probability of survival.