Corrie Painter

HLA-DO acts as a substrate mimic to inhibit HLA-DM by a competitive mechanism

Mammalian class II major histocompatibility (MHCII) proteins bind peptide antigens in endosomal compartments of antigen-presenting cells. The nonclassical MHCII protein HLA-DM chaperones peptide-free MHCII, protecting it against inactivation, and catalyzes peptide exchange on loaded MHCII. Another nonclassical MHCII protein, HLA-DO, binds HLA-DM and influences the repertoire of peptides presented by MHCII proteins. However, the mechanism by which HLA-DO functions is unclear. Here we have used X-ray crystallography, enzyme kinetics and mutagenesis approaches to investigate human HLA-DO structure and function. In complex with HLA-DM, HLA-DO adopts a classical MHCII structure, with alterations near the α subunits 310 helix. HLA-DO binds to HLA-DM at the same sites implicated in MHCII interaction, and kinetic analysis showed that HLA-DO acts as a competitive inhibitor. These results show that HLA-DO inhibits HLA-DM function by acting as a substrate mimic, and the findings also limit the possible functional roles for HLA-DO in antigen presentation.

Model for the peptide-free conformation of class II MHC proteins

Background Major histocompatibility complex proteins are believed to undergo significant conformational changes concomitant with peptide binding, but structural characterization of these changes has remained elusive. Methodology/Principal Findings Here we use molecular dynamics simulations and experimental probes of protein conformation to investigate the peptide-free state of class II MHC proteins. Upon computational removal of the bound peptide from HLA-DR1-peptide complex, the α50-59 region folded into the P1-P4 region of the peptide binding site, adopting the same conformation as a bound peptide. Strikingly, the structure of the hydrophobic P1 pocket is maintained by engagement of the side chain of Phe α54. In addition, conserved hydrogen bonds observed in crystal structures between the peptide backbone and numerous MHC side chains are maintained between the α51-55 region and the rest of the molecule. The model for the peptide-free conformation was evaluated using conformationally-sensitive antibody and superantigen probes predicted to show no change, moderate change, or dramatic changes in their interaction with peptide-free DR1 and peptide-loaded DR1. The binding observed for these probes is in agreement with the movements predicted by the model. Conclusion/Significance This work presents a molecular model for peptide-free class II MHC proteins that can help to interpret the conformational changes known to occur within the protein during peptide binding and release, and can provide insight into possible mechanisms for DM action.

Conformational lability in the class II MHC 310 helix and adjacent extended strand dictate HLA-DM susceptibility and peptide exchange

HLA-DM is required for efficient peptide exchange on class II MHC molecules, but its mechanism of action is controversial. We trapped an intermediate state of class II MHC HLA-DR1 by substitution of αF54, resulting in a protein with increased HLA-DM binding affinity, weakened MHC-peptide hydrogen bonding as measured by hydrogen-deuterium exchange mass spectrometry, and increased susceptibility to DM-mediated peptide exchange. Structural analysis revealed a set of concerted conformational alterations at the N-terminal end of the peptide-binding site. These results suggest that interaction with HLA-DM is driven by a conformational change of the MHC II protein in the region of the α-subunit 310 helix and adjacent extended strand region, and provide a model for the mechanism of DM-mediated peptide exchange.

Conformational variation in structures of classical and non-classical MHCII proteins and functional implications

Recent structural characterizations of classical and non‐classical major histocompatibility complex class II (MHCII) proteins have provided a view into the dynamic nature of the MHCII–peptide binding groove and the role that structural changes play in peptide loading processes. Although there have been numerous reports of crystal structures for MHCII–peptide complexes, a detailed analysis comparing all the structures has not been reported, and subtle conformational variations present in these structures may not have been fully appreciated. We compared the 91 MHCII crystal structures reported in the PDB to date, including an HLA‐DR mutant particularly susceptible to DM‐mediated peptide exchange, and reviewed experimental and computational studies of the effect of peptide binding on MHCII structure. These studies provide evidence for conformational lability in and around the α‐subunit 3‐10 helix at residues α48‐51, a region known to be critical for HLA‐DM‐mediated peptide exchange. A biophysical study of MHC–peptide hydrogen bond strengths and a recent structure of the non‐classical MHCII protein HLA‐DO reveal changes in the same region. Conformational variability was observed also in the vicinity of a kink in the β‐subunit helical region near residue β66 and in the orientation and loop conformation in the β2 Ig domain. Here, we provide an overview of the regions within classical and non‐classical MHCII proteins that display conformational changes and the potential role that these changes may have in the peptide loading/exchange process.

Identification and characterization of T reg–like cells in zebrafish

Regulatory T (T reg) cells are a specialized sublineage of T lymphocytes that suppress autoreactive T cells. Functional studies of T reg cells in vitro have defined multiple suppression mechanisms, and studies of T reg–deficient humans and mice have made clear the important role that these cells play in preventing autoimmunity. However, many questions remain about how T reg cells act in vivo. Specifically, it is not clear which suppression mechanisms are most important, where T reg cells act, and how they get there. To begin to address these issues, we sought to identify T reg cells in zebrafish, a model system that provides unparalleled advantages in live-cell imaging and high-throughput genetic analyses. Using a FOXP3 orthologue as a marker, we identified CD4-enriched, mature T lymphocytes with properties of T reg cells. Zebrafish mutant for foxp3a displayed excess T lymphocytes, splenomegaly, and a profound inflammatory phenotype that was suppressed by genetic ablation of lymphocytes. This study identifies T reg–like cells in zebrafish, providing both a model to study the normal functions of these cells in vivo and mutants to explore the consequences of their loss.

Zebrafish as a platform to study tumor progression.

The zebrafish has emerged as a powerful model system to study human diseases, including a variety of neoplasms. Principal components that have contributed to the rise in use of this vertebrate model system are its high fecundity, ease of genetic manipulation, and low cost of maintenance. Vital imaging of the zebrafish is possible from the transparent embryonic stage through adulthood, the latter enabled by a number of mutant lines that ablate pigmentation. As a result, high-resolution analyses of tumor progression can be accomplished in vivo. Straightforward transgenesis of zebrafish has been employed to develop numerous tumor models that recapitulate many aspects of human neoplastic disease, both in terms of pathologic and molecular conservation. The small size of zebrafish embryos has enabled screens for novel chemotherapeutic agents. Its facile genetics have been exploited in studies that extend beyond modeling cancer to investigations that define new cancer genes and mechanisms of cancer progression. Together, these attributes have established the zebrafish as a robust and versatile model system for investigating cancer. In this chapter we describe methods that are used to study a genes impact on melanoma progression. We detail methods for making transgenic animals and screening for tumor onset as well as methods to investigate tumor invasion and propagation.

Abstract 1273: Adaptive immunity in a zebrafish model of melanoma.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC The recent success of the anti-CLTA-4 antibody, ipilimumab, for late stage metastatic melanoma, provides proof of principle that stimulating the immune system can have profound effects on patient outcome. However, a detailed understanding of the role of immune-modulating cells within the tumor microenvironment has been confounded by the lack of a suitable model system that would allow for direct visualization of cells within the intact tumor microenvironment. Previous work has demonstrated that the zebrafish, Danio Rerio, can be genetically manipulated to produce melanomas that recapitulate BRAFV600E;P53-/- human tumors. Due to its translucency, ease of genetic manipulation and high fecundity, the zebrafish is an attractive model system that allows for in vivo characterization of cells within intact tissue. Using this model system, we have begun investigating the adaptive immune response to melanoma in zebrafish. We have focused our initial investigations on determining whether transcript expression of key adaptive immune-modulatory proteins are present in the zebrafish and to what extent they are expressed intratumorally . Our studies have found that orthologs of CLTA-4, FOXP3, and CD8 are all expressed in wild-type zebrafish, but not in rag-/- zebrafish. Furthermore, transcript levels of CTLA-4 were greatest in the melanoma tumor relative to other regions of the zebrafish, which is consistent with the constitutive expression of CTLA-4 in human melanomas. Additionally, we have cloned the promoters for zebrafish FOXP3 and CD8 and are developing fluorescent reporter lines that will enable us to visualize and ablate subpopulations of T cells within the tumor. These lines, along with previously described reporter lines for MHC II and lck, will enable us to determine the role of adaptive immune cells during melanoma onset and progression in the zebrafish. Our research supports the notion that there is conservation between the human and zebrafish adaptive immune systems, and that similar immune evasion mechanisms may be contributing to melanoma growth in zebrafish. These data position the zebrafsh as a powerful model system for immunological investigations that focus on the adaptive immune response to melanoma. Citation Format: Corrie A. Painter, Craig Ceol. Adaptive immunity in a zebrafish model of melanoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1273. doi:10.1158/1538-7445.AM2013-1273