Rance E. Berg

IL-23 Is Required for Protection against Systemic Infection with Listeria monocytogenes

Listeria monocytogenes (LM) is a Gram-positive, intracellular bacterium that can induce spontaneous abortion, septicemia, and meningitis. Although it is known that neutrophils are required for elimination of the bacteria and for survival of the host, the mechanisms governing the recruitment of neutrophils to LM-infected tissues are not fully understood. We demonstrate here that IL-23 and the IL-17 receptor A (IL-17RA), which mediates both IL-17A and IL-17F signaling, are necessary for resistance against systemic LM infection. LM-infected IL-23p19 knockout (KO) mice have decreased production of IL-17A and IL-17F, while IFN-γ production is not altered by the lack of IL-23. LM induces the production of IL-17A from γδ T cells, but not CD4, CD8, or NK cells. Furthermore, a lack of efficient neutrophil recruitment to the liver is evident in both IL-23p19 KO and IL-17RA KO mice during LM infection. Immunocytochemical analysis of infected livers revealed that neutrophils were able to localize with LM in IL-23p19 KO and IL-17RA KO mice, indicating that IL-23 and IL-17RA do not regulate the precise localization of neutrophils with LM. The importance of IL-23-induced IL-17A was demonstrated by injecting IL-23p19 KO mice with recombinant IL-17A. These mice had reduced LM bacterial burdens compared with IL-23p19 KO mice that did not receive IL-17A. These results indicate that during LM infection, IL-23 regulates the production of IL-17A and IL-17F from γδ T cells, resulting in optimal liver neutrophil recruitment and enhanced bacterial clearance.

Positive selection of an H2-M3 restricted T cell receptor.

Thymocytes are positively selected for alphabeta T cell antigen receptors (TCR) that recognize antigen in conjunction with self-major histocompatibility complex (MHC) molecules. MHC bound peptides participate in positive selection; however, their role has remained controversial. A TCR transgenic mouse was established using a TCR restricted to the MHC class Ib molecule, H2-M3. Having defined H2-M3 as the positively selecting MHC molecule, the severely limited number of H2-M3 binding peptides allowed us to characterize an NADH dehydrogenase subunit 1 (ND1)-derived peptide as the physiological ligand of positive selection. This peptide bears no apparent sequence homology to the cognate peptide, is expressed ubiquitously, and yet does not interfere with peripheral T cells. Our studies also suggest that positive selection becomes promiscuous at high epitope densities.

A Physiological Ligand of Positive Selection Is Recognized as a Weak Agonist

Positive selection is a process that ensures that peripheral T cells express TCR that are self-MHC restricted. This process occurs in the thymus and requires both self-MHC and self-peptides. We have recently established a TCR transgenic (TCRtrans+) mouse model using the C10.4 TCR restricted to the MHC class Ib molecule, H2-M3. Having defined H2-M3 as the positively selecting MHC molecule, the severely limited number of H2-M3 binding peptides allowed us to characterize a mitochondrial NADH dehydrogenase subunit 1-derived 9-mer peptide as the physiological ligand of positive selection. Here, we demonstrate that the NADH dehydrogenase subunit 1 self-peptide is seen by mature C10.4 TCRtrans+ T cells as a weak agonist and induces positive selection at a defined concentration range. We also found that the full-length cognate peptide, a strong agonist for mature C10.4 TCRtrans+ T cells, initiated positive selection, albeit at significantly lower concentrations. At increased peptide concentrations, and thus increased epitope densities, either peptide only induced the development of partially functional T cells. We conclude that successful positive selection only proceeded at a defined, yet fairly narrow window of avidity.

A Physiological Ligand of Positive Selection Is Seen with High Specificity

Positive selection is a process that ensures that peripheral T cells express TCR that are restricted to self-MHC molecules. This process requires both self-MHC and self-peptides. We have recently established a TCR transgenic mouse model (C10.4 TCRtrans+) in which the transgenic TCR was selected on the nonclassical MHC class Ib molecule H2-M3 in conjunction with a physiologically occurring peptide derived from the mitochondrial NADH-dehydrogenase subunit 1 gene (9-mer peptide). Here, the specificity of positive selection of C10.4 TCRtrans+ T cells was examined using a fetal thymic organ culture system. We demonstrated that at low peptide concentrations, shortening the NADH-dehydrogenase subunit 1 gene 9-mer peptide or mutating its surface-exposed side chains severely impaired its ability to induce positive selection. We concluded that under physiological conditions positive selection of C10.4 TCRtrans+ T cells was highly specific and occurred at low epitope densities.