Processing and presentation of an antigen of Mycobacterium avium require access to an acidified compartment with active proteases

Diet modulates colonic T cell responses by regulating the expression of a Bacteroides thetaiotaomicron antigen

T cell responses to symbionts in the intestine drive tolerance or inflammation depending on the genetic background of the host. These symbionts in the gut sense the available nutrients and adapt their metabolic programs to use these nutrients efficiently. Here, we ask whether diet can alter the expression of a bacterial antigen to modulate adaptive immune responses. We generated a CD4⁺ T cell hybridoma, BθOM, specific for Bacteroides thetaiotaomicron (B. theta). Adoptively transferred transgenic T cells expressing the BθOM TCR proliferated in the colon, colon-draining lymph node, and spleen in B. theta-colonized healthy mice and differentiated into regulatory T cells (T_regs) and effector T cells (T_effs). Depletion of B. theta-specific T_regs resulted in colitis, showing that a single protein expressed by B. theta can drive differentiation of T_regs that self-regulate T_effs to prevent disease. We found that BθOM T cells recognized a peptide derived from a single B. theta protein, BT4295, whose expression is regulated by nutrients, with glucose being a strong catabolite repressor. Mice fed a high-glucose diet had a greatly reduced activation of BθOM T cells in the colon. These studies establish that the immune response to specific bacterial antigens can be modified by changes in the diet by altering antigen expression in the microbe.

Phagocytic processing of antigens for presentation by class II major histocompatibility complex molecules

Microbes and other particulate antigens (Ags) are internalized by phagocytosis and then reside in plasma membrane-derived phagosomes. The contribution of phagosomes to the degradation of Ags has long been appreciated. It has been unclear, however, whether peptides derived from these degraded antigens bind class II major histocompatibility complex (MHC-II) molecules within phagosomes or within endocytic compartments that receive Ag fragments from phagosomes. Recent experiments have demonstrated that phagosomes containing Ag-conjugated latex beads express a full complement of Ag-processing molecules, e.g. MHC-II molecules, invariant chain, H2-DM and proteases sufficient to degrade bead- associated Ag. These phagosomes mediate the formation of peptide-MHC-II complexes, which are transported to the cell surface and presented to T cells. Phagosomes acquire both newly synthesized and plasma membrane-derived MHC-II molecules, but the formation of peptide-MHC-II complexes in phagosomes primarily involves newly synthesized MHC-II molecules. The content and traffic of phagosomal proteins vary considerably with the type of Ag ingested. Pathogenic microbes can alter phagosome composition and function to reduce Ag processing. For example, Mycobacterium tuberculosis blocks the maturation of phagosomes and reduces the ability of infected cells to present exogenous soluble protein Ags.

Phagocytic antigen processing and effects of microbial products on antigen processing and T-cell responses

Processing of exogenous antigens and microbes involves contributions by multiple different endocytic and phagocytic compartments. During the processing of soluble antigens, different endocytic compartments have been demonstrated to use distinct antigen-processing mechanisms and to process distinct sets of antigenic epitopes. Processing of particulate and microbial antigens involves phagocytosis and functions contributed by phagocytic compartments. Recent data from our laboratory demonstrate that phagosomes containing antigen-conjugated latex beads are fully competent class II MHC (MHC-II) antigen-processing organelles, which generate peptide:MHC-II complexes. In addition, phagocytosed antigen enters an alternate class I MHC (MHC-I) processing pathway that results in loading of peptides derived from exogenous antigens onto MHC-I molecules, in contrast to the cytosolic antigen source utilized by the conventional MHC-I antigen-processing pathway. Antigen processing and other immune response mechanisms may be activated or inhibited by microbial components to the benefit of either the host or the pathogen. For example, antigen processing and T-cell responses (e.g. Th1 vs Th2 differentiation) are modulated by multiple distinct microbial components, including lipopolysaccharide, cholera toxin, heat labile enterotoxin of Escherichia coli, DNA containing CpG motifs (found in prokaryotic and invertebrate DNA but not mammalian DNA) and components of Mycobacterium tuberculosis.

Growth within macrophages increases the efficiency of Mycobacterium avium in invading other macrophages by a complement receptor-independent pathway

Infections caused by organisms of the Mycobacterium avium complex occur in approximately 50 to 60% of patients with AIDS. M. avium is an intracellular pathogen that survives and multiplies within mononuclear phagocytes. In this study, we investigated the uptake of M. avium grown within macrophages (intracellular growth M. avium [IG]) by a second macrophage compared with M. avium cultured in broth (extracellular growth M. avium [EG]). The results showed that IG was six- to eightfold more efficient than EG in entering macrophages. In addition, while an anti-CR3 antibody was able to inhibit approximately 60% of EG uptake by macrophages, it failed to inhibit the entry of IG. In contrast to EG, IG uptake into macrophages was significantly inhibited in the presence of anti-beta1-integrin and anti-transferrin receptor antibodies. Entry into macrophages by alternate receptors was associated with resistance to tumor necrosis factor alpha (TNF-alpha) stimulation. While stimulation with TNF-alpha resulted in inhibition of the growth of EG, it was not associated with inhibition of intracellular growth of IG. Investigation of the reason why M. avium is able to sense the changes in the intracellular environment triggering a change to the invasive phenotype suggests a direct relationship with macrophage apoptosis. These results suggest that intracellular growth is associated with novel mechanisms of M. avium uptake of macrophages and that those mechanisms appear to offer advantages to the bacteria in escaping the host defense.