Mycobacterium tuberculosis, the Tuberculosis-causative pathogen identified 142 years ago, remains a global challenge, currently infecting at least a quarter of the world's population1. Transmitted through airborne droplets from infected people, M. tuberculosis reaches alveolar macrophages in the lungs where it can survive for long periods in a latent state2,3. Not only is the local macrophagic response activated, but in recent years, it has been described that peripheral monocytes can be recruited to the respiratory tract and differentiate into alveolar macrophages, generating an even more robust response against M. tuberculosis than their counterpart of fetal origin2,4.
Macrophages are fundamental players of innate immunity. Upon M. tuberculosis ingestion, macrophages display numerous microbicidal functions, such as the secretion of proinflammatory cytokines, the fusion of the phagolysosome, and the activation of other immune cells in order to kill M. tuberculosis2,3. However, the complex architectural structure of this pathogen provides effectors (e.g., proteins and lipids) capable of regulating macrophage metabolism and functions and, in consequence, the inflammatory process3. This manipulation of macrophage responses, through the secretion of virulence factors or the exploitation of host factors, leads to different escape strategies exerted by M. tuberculosis. Some key evasion mechanisms include induction of anti-inflammatory cytokines, inhibition of phagolysosome maturation and acidification, oxidative stress alterations, autophagy disruption, and deficiencies during antigen processing and presentation3,5.
A tightly regulated interaction between macrophages and M. tuberculosis is crucial for the development of a proper immune response. Therefore, studying these synapses is key to identifying immunoprotective or immunopathogenic mechanisms induced by host-pathogen crosstalk, as well as identifying potential therapeutic targets. Many receptors mediate the recognition and/or internalization of M. tuberculosis, including TLRs6,7,8, NLRs7,8, complement receptors6,8, C-type lectin receptors6,8, and scavenger receptors6,8. Accumulating data indicate that both surface-bound and intracellular PRRs play an important role during infection, recognizing opsonized and non-opsonized M. tuberculosis.
Zihad and Sifat et al. have recently reviewed the participation of the PRRs in M. tuberculosis-induced responses by innate cells9. In particular, most members of the TLR family have been implicated in the interaction with M. tuberculosis ligands6,7. Surface TLR2 recognizes mycobacterial antigens such as acylated lipoproteins, 19-kDa lipoprotein, LprA lipoprotein, LprG lipoprotein, 30-kDa antigen, 38-kDa antigen, MymA, proline-proline-glutamic acid (PPE)-57, LAM, LM, PIM, heat shock protein 60, signature protein Rv1509 and the secreted protein ESAT-67. Membrane TLR4 interacts with heat shock proteins, 38-kDa antigen, RpfE, Rv0652, Rv0335c, Rv2659c, Rv1738, Rv2627c, Rv2628, GrpE, and HBHA. On the other hand, ligands for endosomal TLRs (TLR 3, 7, 8, and 9) include dsRNA, tRNA, ssRNA, phagosomal RNA, and dsDNA of M. tuberculosis. In addition, TLR2 has an active role in initiating immune responses when acting in conjunction with TLR1 and TLR6, TLR4, and TLR96,7. NOD2 and NLRP3 are the most well-characterized cytoplasmic NLRs in Tuberculosis. Although their specific ligands remain under study, these receptors are activated by muramyl dipeptide or ESAT-6, respectively7,8. C-type lectin receptors are classically involved in M. tuberculosis endocytosis. While Dectin-2 acts as a direct PRR for ManLAM of M. tuberculosis cell wall, the Dectin-1 ligand has not been discovered yet8. Mincle and MCL both recognize the glycolipid trehalose-6,6′-dimycolate (TDM), also called the cord factor7. DCAR interacts with the mycobacterial glycolipids phosphatidyl-myo-inositol mannosides (PIMs)7. CR3 recognizes mycobacterial LAM and PIM, DC-SIGN ligates ManLAM, MR recognizes a number of M. tuberculosis components, including ManLAM, PIM, LM, 38-kDa glycoprotein, 19-kDa antigen, and other mannosylated proteins while DCIR ligand is still unidentified.
Soluble CLRs include SP-A, which recognizes ManLam, LM, 60-kDa glycoprotein, and glycoprotein Apa; SP-D interacting with LAM, LM, and PILAM; and MBL, which specializes in ManLAM recognition6,7. Scavenger receptors are also phagocytic PRRs. SR-A and MARCO have TDM as their ligand, SR-B1 recognizes ESAT-6, and CD36 binds ManLAM and LM7,8. Additionally, Dectin-1, Mincle, and MARCO can also combine with TLR2 or TLR4 to trigger signals after detecting M. tuberculosis PAMPs6,7. CD14 is a surface receptor that has the ability to internalize nonopsonized bacteria and also recognizes the heat shock protein Chaperonin 60.1. In particular, CD14 functions as a co-receptor along with MARCO and TLR26. AIM2 is a cytosolic receptor that can sense ssDNA upon M. tuberculosis escapes from the phagosome8. Finally, AhR is a ligand-activated transcription factor that binds pigmented virulence factor naphthoquinone phthiocol from M. tuberculosis6.
Many of the interactions described above have been postulated and not strictly demonstrated. Even the ligands for certain receptors remain unknown, which reinforces the need to better understand the field of M. tuberculosis immunorecognition. In this context, the costimulatory molecule SLAMF1 (Signaling Lymphocytic Activation Molecule) has been recently described as a M. tuberculosis receptor by Barbero et al.10. By acting not only as a signaling molecule but also as a M. tuberculosis sensor, SLAMF1 has a particularly intriguing role in Tuberculosis. SLAMF1 can induce the activation of immune cells by modulating protective functions such as the production of IFN-γ by T cells through Erk/CREB phosphorylation11,12,13, autophagy in neutrophils14, and bacterial clearance in macrophages15.
Receptors-ligand interactions have been studied over the years using techniques such as ELISA, Surface Plasmon Resonance (SPR), Isothermal Titration Calorimetry (ITC), Fluorescence Polarization (FP), X-ray Crystallography, Nuclear Magnetic Resonance (NMR) Spectroscopy, Microscale Thermophoresis (MST), Resonance Energy Transfer (e.g., BRET or FRET) Confocal Microscopy, Electron Microscopy, Cryo-Electron Microscopy (Cryo-EM) and Atomic Force Microscopy (AFM)16,17,18,19,20,21,22,23,24,25,26,27,28. Some of these approaches imply the use of reporter genes, labeled recombinant proteins or chimeric molecules, knockout, knockdown, or overexpression models. Alternatively, computational tools can predict receptor-ligand interactions and binding sites and are often used in combination with biological approaches to gain a comprehensive understanding of the interactions29,30,31. Here, two alternatives to detect biochemical interaction and also a section on how to fluorescently label bacteria are described. A protocol that allows the study of receptor-pathogen interactions in vitro is presented, particularly evaluating the SLAMF1-M. tuberculosis engagement through flow cytometry and fluorescence microscopy, two usually available and routinely used techniques.