Tuberculosis (TB) continues to be the leading cause of death worldwide from a single infectious agent, with an estimated 7.5 million new cases reported in 2023 (WHO, 2023). The emergence of multidrug-resistant TB (MDR-TB), primarily caused by Mycobacterium tuberculosis (Mtb) strains resistant to the two most potent anti-TB drugs, isoniazid (INH) and rifampin (RFP), poses a significant threat to TB control and elimination efforts. The alternative treatment for MDR-TB involves second-line drugs, which are not only more expensive but also more toxic and less effective. Consequently, MDR-TB patients require longer treatment durations to achieve negative acid-fast bacilli (AFB) sputum results (Palomiño et al., 2007).

In Argentina, 12,569 TB cases were reported in 2021 (Instituto Nacional de Enfermedades Respiratorias Dr. Emilio Coni, 2023), with a rate of 0.95 RFP-resistant and MDR-TB cases per 100,000 inhabitants (World Health Organization, 2022). Notably, the Mtb strain known as "M," belonging to the Haarlem 2 family genotype SIT 2, was responsible for the largest and most prolonged outbreak of MDR-TB in Latin America (Ritacco et al., 2012). Although its prevalence has started to decline (Monteserin et al., 2019), it still accounts for 25 % of extensively drug-resistant TB (XDR-TB) cases. In contrast, strain 410, a clonal variant of strain M, has caused only a single TB case since the onset of the outbreak (Yokobori et al., 2013). Thus, considering as pathogen's reproductive fitness the number of secondary cases it generates (Borrell and Gagneux, 2009), M and 410 strains have notable differences in their relative fitness.

The host immune response is a critical evolutionary force in shaping Mtb evolution (Brites and Gagneux, 2012). Therefore, some of the differences in relative fitness among Mtb strains may be attributed to their varying abilities to evade the host immune system. In this context, M strain has demonstrated an ability to manipulate both the innate and adaptive host immune responses for its own benefit (Yokobori et al., 2022). Unlike H37Rv and 410 strains, M induces non-functional CD8+ cytotoxic T cells in both TB patients and healthy PPD+ BCG-vaccinated donors (Geffner et al., 2009), as result of subtle changes in CD69 signaling and impaired CD4+ T cell help due to reduced IL-2 secretion (Geffner et al., 2014). The diminished cytotoxic T lymphocyte (CTL) activity induced by M may allow the bacteria to evade the immune system and persist within the host.

Increasing evidence highlights the significance of bacterial cell wall components in the interaction between Mtb and the host (Garcia-Vilanova et al., 2019, Queiroz and Riley, 2017). Polymorphisms in promoters and genes involved in cell wall biogenesis may affect the effectiveness of different Mtb strains in modulating the host immune response (Almeida et al., 2023, Briken et al., 2004). In this context, we previously identified polymorphisms in 11 proteins and 5 intergenic regions occurring between M and 410 strains (Bigi et al., 2017). Some of these polymorphisms are located in proteins or promoter regions associated with cell wall component metabolism, while others are found in proteins associated with drug resistance (Bigi et al., 2017). Additionally, comparative protein and lipid analysis of both strains reveal that M displays a distinctive lipid profile in its cell envelope (Bei et al., 2020). Mycobacterial cell envelope lipids play crucial roles in penetration, replication, and survival within host cells (Queiroz and Riley, 2017), as well as in evading the host immune response through various mechanisms.

In this study, we aimed to assess the impact of extractable lipids derived from wild M, 410, CD1551 and H37Rv strains, as well as from mutants in genes Rv1861, Rv0503, Rv2187, Rv3193, and Rv3787c that are polymorphic between M and 410 strains, on the activation and functionality of T cells from PPD+ healthy individuals.