Interferon-gamma listericidal action is mediated by novel Rab5a functions at the phagosomal environment

Control and clearance of Listeria monocytogenes infection is an interferon-gamma-dependent process. The listericidal mechanism of action involves activation of NADPH oxidase and inducible nitric-oxide synthase to produce reactive oxygen and nitrogen intermediate radicals, respectively. Recently, we have described in a nonpathogenic model of L. monocytogenes (hemolysin negative mutant strain) that the interferon-gamma-inducible GTPase Rab5a contributed to Listeria destruction in resting macrophages. Here, we report in a pathogenic model of L. monocytogenes (hemolysin-positive strain) that Rab5a plays a central role in Listeria destruction induced by interferon-gamma and within the phagosomal environment. These findings reveal the importance of Rab5a as the responsible factor mediating the listericidal action of interferon-gamma. Active Rab5a causes remodeling of the phagosomal environment, facilitates the translocation of Rac2 to LM phagosomes, and regulates the activity of this GTPase. Rac2 activation and translocation governs the phagocyte NADPH oxidase activity and the consequent reactive oxygen intermediate production that leads to killing of the pathogen.

The lack of consensus for I-A(g7)-peptide binding motifs: is there a requirement for anchor amino acid side chains?

We discuss here the problems in identifying sequence motifs of peptides that bind to I-A(g7), the class II histocompatibility molecule of NOD diabetic mice. We present studies that indicate a minor contribution of amino acid side chains for binding. A peptide from the Ealpha chain binds to I-A(g7) molecules and is recognized by CD4 T cells. By producing single-residue mutations we identified four residues that were considered to contact the T cell receptor. No residue was found to be essential for binding to I-A(g7): a peptide that contained the T cell contact residues, on a backbone of alanines, bound to I-A(g7) and stimulated the T cells. We conclude that peptides can bind to I-A(g7) without the requirement for residues with prominent side chains to anchor them.

Autoreactivity of T cells from nonobese diabetic mice: an I-Ag7-dependent reaction

Mice bearing the I-Ag7 class II major histocompatibility complex molecules contain a high number of spontaneous autoreactive T cells, as estimated by limiting-dilution assays. We found this autoreactivity in various strains that bear the I-Ag7 molecule, such as the nonobese diabetic (NOD) mouse strain, which spontaneously develops autoimmune diabetes. However, NOD mice strains that do not express the I-Ag7 molecule, but instead express I-Ab, do not have a high incidence of autoreactive T cells. About 15% of the autoreactive T cells also recognize the I-Ag7 molecule expressed in the T2 line, which is defective in the processing of protein antigens. We interpret this to mean that some of the T cells may interact with class II molecules that are either devoid of peptides or contain a limited peptide content. We also find a high component of autoreactivity among antigen-specific T cell clones. These T cell clones proliferate specifically to protein antigens but also have a high level of reactivity to antigen-presenting cells not pulsed with antigen. Thus, the library of T cell receptors in NOD mice is skewed to autoreactivity, which we speculate is based on the weak peptide-binding properties of I-Ag7 molecules.

The class II MHC I-Ag7 molecules from non-obese diabetic mice are poor peptide binders

The class II molecules of the diabetes-prone NOD mice, I-Ag7, showed very limited amounts of stable form when analyzed by SDS-PAGE. We included the analysis of spleen B cells and B lymphoma cells transfected with I-Ag7 genes. Early during bio-synthesis there was invariant chain binding to the alpha beta-chains. Examination of APCs from F1 mice (NOD x C57BL/6) indicated that the same APC expressed high levels of unstable I-Ag7 and normal amounts of stable class II molecules compared with the other haplotype (I-Ab). The half-life of I-Ag7-peptide complexes on the cell surface of APC was significantly shorter than that of other class II haplotypes. Direct biochemical demonstration of peptide interactions with I-Ag7 was difficult to demonstrate. In T cell assays, the immunogenic peptides, including the diabetogenic Ag, were rapidly lost when peptide-pulsed APCs were washed free of peptide. We hypothesize that the weak and unstable peptide-binding property of I-Ag7 molecules does not favor the elimination or inactivation of autoreactive T cells.

The class II MHC I-Ag7 molecules from non-obese diabetic mice are poor peptide binders

The class II molecules of the diabetes-prone NOD mice, I-Ag7, showed very limited amounts of stable form when analyzed by SDS-PAGE. We included the analysis of spleen B cells and B lymphoma cells transfected with I-Ag7 genes. Early during bio-synthesis there was invariant chain binding to the alpha beta-chains. Examination of APCs from F1 mice (NOD x C57BL/6) indicated that the same APC expressed high levels of unstable I-Ag7 and normal amounts of stable class II molecules compared with the other haplotype (I-Ab). The half-life of I-Ag7-peptide complexes on the cell surface of APC was significantly shorter than that of other class II haplotypes. Direct biochemical demonstration of peptide interactions with I-Ag7 was difficult to demonstrate. In T cell assays, the immunogenic peptides, including the diabetogenic Ag, were rapidly lost when peptide-pulsed APCs were washed free of peptide. We hypothesize that the weak and unstable peptide-binding property of I-Ag7 molecules does not favor the elimination or inactivation of autoreactive T cells.

Relationship between beta cell injury and antigen presentation in NOD mice

We have made three observations that are important for our understanding of the dynamics of presentation of diabetogenic Ags in immunologically induced diabetes. First, the APC in the islets of Langerhans were found normally to contain diabetogenic peptides on their I-Ag7 molecules. This was found in freshly harvested islet cells from prediabetic NOD mice and, importantly, from NOD.SCID mice. Second, the presence of diabetogenic lymphocytes improved the presenting function of intra-islet APC. We interpret this to mean that lymphocytes can regulate intra-islet APC function before the development of diabetes. Finally, spleen APC can be found bearing diabetogenic Ag after acute injury to islets. These APC may be important in lymphocyte stimulation outside the environment of the pancreas.

Relationship between beta cell injury and antigen presentation in NOD mice

We have made three observations that are important for our understanding of the dynamics of presentation of diabetogenic Ags in immunologically induced diabetes. First, the APC in the islets of Langerhans were found normally to contain diabetogenic peptides on their I-Ag7 molecules. This was found in freshly harvested islet cells from prediabetic NOD mice and, importantly, from NOD.SCID mice. Second, the presence of diabetogenic lymphocytes improved the presenting function of intra-islet APC. We interpret this to mean that lymphocytes can regulate intra-islet APC function before the development of diabetes. Finally, spleen APC can be found bearing diabetogenic Ag after acute injury to islets. These APC may be important in lymphocyte stimulation outside the environment of the pancreas.

Role of complement component C1q in phagocytosis of Listeria monocytogenes by murine macrophage-like cell lines

Listeria monocytogenes is a facultative intracellular pathogen of a great variety of cells. Among them, macrophages constitute the major effector cells of listerial immunity during the course of an infection. Although the molecular bases of L. monocytogenes attachment and entry to phagocytes are not completely understood, it has been demonstrated that C3b significantly increases L. monocytogenes uptake by macrophages via complement receptor type 3. The first component of complement, C1q, is present in organic fluids at a relatively high concentration, and C1q receptor sites in macrophages are also abundant. In the present report, results of studies on the role of C1q in the internalization and infectivity of L. monocytogenes by macrophages are presented. L. monocytogenes uptake is enhanced by prior treatment of bacteria with normal sera. Heated serum or C1q-deficient serum abrogates this enhancement. Purified C1q specifically restored uptake. This effect was blocked by the addition of F(ab')2 anti-C1q antibody but not by an irrelevant matched antibody. Direct binding of C1q to L. monocytogenes was specific, saturable, and dose dependent with both fluorescent and radiolabeled C1q. N-Acetyl-D-alanyl-L-isoglutamine, diaminopimelic acid, and L-rhamnose caused a significant dose-dependent inhibition of C1q binding to bacteria, suggesting that these molecules, at least, are involved in the attachment of C1q to L. monocytogenes cell wall. When C1q binding structures on macrophage-like cells were blocked with saturating concentrations of C1q, the uptake of C1q-opsonized bacteria was less than in untreated cells. These experiments demonstrate that, in addition to other reported mechanisms, L. monocytogenes binds C1q, which mediates enhanced uptake by macrophages through C1q binding structures.