The impact of drug resistance mutations induced by nucleoside reverse transcriptase (RT) inhibitors (NRTI) on cytotoxic T-lymphocyte (CTL) recognition of human immunodeficiency virus type 1 strain LAI (HIV-1 LAI )RT was addressed in 35 treated or untreated patients. Two HIV-1 LAI RT regions encompassing mutation M41L, L74V, M184V, and T215Y/F were recognized in 75 and 83% mutated and in 33 and 42% unmutated samples, respectively. A total of 41 new CTL epitopes overlapping these mutations were predicted. Mutations enhanced HLA-binding scores of 17 epitopes, decreased scores of 5, and had no effect in 19. Four predicted epitopes containing mutations 41, 74, and 184 were tested and recognized by CD8 cells from mutated or unmutated samples, with frequencies up to 270 gamma interferon spot-forming cells per 10 6 peripheral blood mononuclear cells. Therefore, RT mutations induced by NRTI can increase the immunogenicity of RT for CTL and might allow a better immune control of resistant viruses in vivo, suggesting that specific immune therapy might help prevent these mutations.Cytotoxic T lymphocytes (CTL) specific for human immunodeficiency virus (HIV) or simian immunodeficiency virus are considered the most efficient virus-specific immune responses (4,26,29,39). The strength and the diversity of CTL responses (16, 54) have been proposed, together with reverse transcriptase (RT) infidelity (7,33,37), as an important factor for virus variability at time of asymptomatic disease and strong immune functions. Some viral mutations can decrease immunogenicity by interfering with the intracellular processing or with the HLA binding of viral peptides, thereby resulting in a lack of CTL recognition (5,11,13,14,22,30,32,34). In contrast, new HIV variants that do not interfere with such processes can be immunogenic for specific new CTL clones (16), a fact which contributes to some extent to determining HIV variability (54).The high level of HIV type 1 (HIV-1) RT sequence conservation among different HIV isolates (25) makes RT one of the most frequent targets for CTL recognition; indeed, 80% of HIV-infected individuals have specific RT-specific CTL (17). Prolonged antiviral mono-or bitherapy with nucleoside RT inhibitors (NRTI), however, results in selection of HIV-1 strains containing mutations in the RT gene (36). These mutations often have an impact on the enzymatic activity of RT and on the fitness of the virus (2, 45). These drug-induced mutations are highly standardized and characteristic of the various NRTI used (28, 38). Highly active antiretroviral therapies (HAART) combining various drug regimens have decreased the occurrence of such mutations by reducing levels of virus replication, but they concomitantly decrease the intensity of the HIV-specific CTL responses (10,15,29). Currently viral replication is efficiently controlled in only 50% of patients receiving HAART; frequency of treatment failures is increasing and correlates with high levels of drug-induced mutations (56). In industrialized countries, approximately 15% ...