Olaf Rötzschke

Pool sequencing of natural HLA-DR, DQ, and DP ligands reveals detailed peptide motifs, constraints of processing, and general rules.

We have approached the problem of MHC class II ligand motifs by pool sequencing natural peptides eluted from HLA-DR, DQ, and DP molecules. The results indicate surprisingly clear patterns, although not quite as clear as with natural class I ligands. The most striking feature is a highly dominant Proline at position 2. We interpret this to be a consequence of aminopeptidase N-like activity in processing. Another general aspect is the existence of three to four hydrophobic or aromatic anchors, whereby the first and the last are separated by five to eight residues. The peptide motifs for HLA-DR1, DR5, DQ7, and DPw4 are allele-specific and differ by spacing and occupancy of anchors. The anchors tend to be flanked by clusters of charged residues, and small residues, especially Ala, are frequent in the motif centers. These detailed motifs allow one to interpret most previous (DR-) motifs as fitting one or more of the anchors or conserved clusters. The relative motif symmetry suggests the possibility of bidirectional binding of peptides in the class II groove.

Naturally-occurring peptide antigens derived from the MHC class-I-restricted processing pathway

The extraction of naturally-processed peptides from MHC class I glycoproteins has paved the way for a major advance in the understanding of the antigen processing pathway that ultimately induces cytotoxic T-cell responses. Here, Olaf Rötzschke and Kirsten Falk review these new developments and discuss their findings in terms of a novel hypothesis of MHC class-I-restricted processing.

MHC molecules as peptide receptors

The central unit for regulation of the specific immune system is a trimolecular complex made up of the T cell antigen receptor, the MHC molecule, and the MHC ligand. The third component is a peptide derived as a degradation product from a protein. During recent years there has been some progress in understanding the interaction between MHC molecules and their peptide ligands: MHC molecules are peptide receptors of peculiar specificity, being able to accommodate millions of different peptides provided they share some common features.

Uneven tissue distribution of minor histocompatibility proteins versus peptides is caused by MHC expression.

Naturally processed minor histocompatibility (H) peptides corresponding to H-4b, H-Y, and an unmapped BALB.B minor H gene were quantified in a relative way in 15 different tissues of male BALB.B mice. For one of these minor H antigens, we could also determine the relative content of the respective protein. For each minor H peptide, an individual tissue distribution was found. Tissues expressing little or no MHC (major histocompatibility complex), like brain, contained only small amounts of minor H peptides or none at all, although the same tissues contained minor H protein in substantial quantities. By contrast, Kb-expressing brains from mice transgenic for Kb under control of the glial acidic protein promoter contained both minor H peptide and protein in high amounts. Thus, the expression of minor H peptides in a given tissue is dependent on coexpression of the restricting MHC class I molecules.

Bidirectional binding of invariant chain peptides to an MHC class II molecule

T-cell recognition of peptides bound to MHC class II (MHCII) molecules is a central event in cell-mediated adaptive immunity. The current paradigm holds that prebound class II-associated invariant chain peptides (CLIP) and all subsequent antigens maintain a canonical orientation in the MHCII binding groove. Here we provide evidence for MHCII-bound CLIP inversion. NMR spectroscopy demonstrates that the interconversion from the canonical to the inverse alignment is a dynamic process, and X-ray crystallography shows that conserved MHC residues form a hydrogen bond network with the peptide backbone in both orientations. The natural catalyst HLA-DM accelerates peptide reorientation and the exchange of either canonically or inversely bound CLIP against antigenic peptide. Thus, noncanonical MHC-CLIP displays the hallmarks of a structurally and functionally intact antigen-presenting complex.

Influenza A Virus Infection Results in a Robust, Antigen-Responsive, and Widely Disseminated Foxp3+ Regulatory T Cell Response

ABSTRACT Foxp3+ CD4+ regulatory T cells (Tregs) represent a highly suppressive T cell subset with well-characterized immunosuppressive effects during immune homeostasis and chronic infections, although the role of these cells in acute viral infections is poorly understood. The present study sought to examine the induction of Foxp3+ CD4+ Tregs in a nonlethal murine model of pulmonary viral infection by the use of the prototypical respiratory virus influenza A. We establish that influenza A virus infection results in a robust Foxp3+ CD4+ T cell response and that regulatory T cell induction at the site of inflammation precedes the effector T cell response. Induced Foxp3+ CD4+ T cells are highly suppressive ex vivo, demonstrating that influenza virus-induced Foxp3+ CD4+ T cells are phenotypically regulatory. Influenza A virus-induced regulatory T cells proliferate vigorously in response to influenza virus antigen, are disseminated throughout the site of infection and primary and secondary lymphoid organs, and retain Foxp3 expression in vitro, suggesting that acute viral infection is capable of inducing a foreign-antigen-specific Treg response. The ability of influenza virus-induced regulatory T cells to suppress antigen-specific CD4+ and CD8+ T cell proliferation and cytokine production correlates closely to their ability to respond to influenza virus antigens, suggesting that virus-induced Tregs are capable of attenuating effector responses in an antigen-dependent manner. Collectively, these data demonstrate that primary acute viral infection is capable of inducing a robust, antigen-responsive, and suppressive regulatory T cell response.

Discovery of six new susceptibility loci and analysis of pleiotropic effects in leprosy

Genome-wide association studies (GWAS) have led to the discovery of several susceptibility loci for leprosy with robust evidence, providing biological insight into the role of host genetic factors in mycobacterial infection. However, the identified loci only partially explain disease heritability, and additional genetic risk factors remain to be discovered. We performed a 3-stage GWAS of leprosy in the Chinese population using 8,313 cases and 16,017 controls. Besides confirming all previously published loci, we discovered six new susceptibility loci, and further gene prioritization analysis of these loci implicated BATF3, CCDC88B and CIITA-SOCS1 as new susceptibility genes for leprosy. A systematic evaluation of pleiotropic effects demonstrated a high tendency for leprosy susceptibility loci to show association with autoimmunity and inflammatory diseases. Further analysis suggests that molecular sensing of infection might have a similar pathogenic role across these diseases, whereas immune responses have discordant roles in infectious and inflammatory diseases.

The lymph as a pool of self-antigens.

Prenodal lymph is generated from the interstitial fluid that surrounds organs, and thus contains products of organ metabolism and catabolism. New proteomic analyses of lymph have identified proteins and peptides that are derived from capillary extravasation and tissue-specific proteins. Many of these peptides are detected at nanomolar concentrations in the lymph before passage through a regional lymph node. Before entering the node and once inside, proteins and processed peptides are filtered from the lymph by circulating immature dendritic cells (DCs) or non-activated nodal antigen-presenting cells (APCs) (macrophages, B cells and immature DCs). Here, we suggest that this process ensures organ-specific self-antigens are displayed to circulating and nodal APCs, thus contributing to the maintenance of peripheral tolerance.

Allergic airway diseases in a tropical urban environment are driven by dominant mono‐specific sensitization against house dust mites

Southeast Asian populations are increasingly affected by allergic airway diseases. Etiology and specific causes, however, are still unknown. The aim of this study is therefore to identify allergens and risk factors for the high prevalence of allergic airway disease in the tropical urban environment.

Small Organic Compounds Enhance Antigen Loading of Class II Major Histocompatibility Complex Proteins by Targeting the Polymorphic P1 Pocket

Major histocompatibility complex (MHC) molecules are a key element of the cellular immune response. Encoded by the MHC they are a family of highly polymorphic peptide receptors presenting peptide antigens for the surveillance by T cells. We have shown that certain organic compounds can amplify immune responses by catalyzing the peptide loading of human class II MHC molecules HLA-DR. Here we show now that they achieve this by interacting with a defined binding site of the HLA-DR peptide receptor. Screening of a compound library revealed a set of adamantane derivatives that strongly accelerated the peptide loading rate. The effect was evident only for an allelic subset and strictly correlated with the presence of glycine at the dimorphic position β86 of the HLA-DR molecule. The residue forms the floor of the conserved pocket P1, located in the peptide binding site of MHC molecule. Apparently, transient occupation of this pocket by the organic compound stabilizes the peptide-receptive conformation permitting rapid antigen loading. This interaction appeared restricted to the larger Glyβ86 pocket and allowed striking enhancements of T cell responses for antigens presented by these “adamantyl-susceptible” MHC molecules. As catalysts of antigen loading, compounds targeting P1 may be useful molecular tools to amplify the immune response. The observation, however, that the ligand repertoire can be affected through polymorphic sites form the outside may also imply that environmental factors could induce allergic or autoimmune reactions in an allele-selective manner.