MHC molecules lead many lives. Workshop on MHC Class I Molecules at the interface between Biology & Medicine

Distinctive CD8+ T cell and MHC class I signatures in polycythemia vera patients

Polycythemia vera (PV) is a myeloproliferative neoplasm characterized by overproduction of red blood cells. We have performed a comprehensive characterization of blood immune cells for expression of naïve and memory receptors as well as β₂m-associated and β₂m-free MHC class I heavy chains, also known as closed and open conformers, respectively, in PV patients and age-matched controls (CTR). We show that the peripheral CD3⁺CD8⁺ T cell pool in PV patients is clearly divided into two discrete populations, a more granular CD3⁺CD8^high T cell population enriched in effector-memory CD45RA⁺ T cells (CD8⁺ TEMRA) when compared to CTR (P < 0.001), and a less granular CD3⁺CD8^int T cell population that is completely absent in the CTR group (78 vs. 0%, P < 0.001) and is a mixture of naïve (CD8⁺ T_N) and CD8⁺ TEMRA cells expressing intermediate levels of CD28, i.e., CD3⁺CD8^intCD28^int. While the percentage of CD3⁺CD8^int TN cells correlated positively with the number of erythrocytes, the percentage of CD3⁺CD8^int TEMRA correlated negatively with the number of platelets. Finally, we report that PV patients' lymphocytes and monocytes display lower levels of closed (W6/32⁺) MHC-I conformers at the cell surface while exhibiting increased amounts of open (HC-10⁺) MHC-I conformers. The implications of this distinctive immune signature are discussed.

MHC I Expression Regulates Co-clustering and Mobility of Interleukin-2 and -15 Receptors in T Cells

MHC glycoproteins form supramolecular clusters with interleukin-2 and -15 receptors in lipid rafts of T cells. The role of highly expressed MHC I in maintaining these clusters is unknown. We knocked down MHC I in FT7.10 human T cells, and studied protein clustering at two hierarchic levels: molecular aggregations and mobility by Förster resonance energy transfer and fluorescence correlation spectroscopy; and segregation into larger domains or superclusters by superresolution stimulated emission depletion microscopy. Fluorescence correlation spectroscopy-based molecular brightness analysis revealed that the studied molecules diffused as tight aggregates of several proteins of a kind. Knockdown reduced the number of MHC I containing molecular aggregates and their average MHC I content, and decreased the heteroassociation of MHC I with IL-2Rα/IL-15Rα. The mobility of not only MHC I but also that of IL-2Rα/IL-15Rα increased, corroborating the general size decrease of tight aggregates. A multifaceted analysis of stimulated emission depletion images revealed that the diameter of MHC I superclusters diminished from 400-600 to 200-300 nm, whereas those of IL-2Rα/IL-15Rα hardly changed. MHC I and IL-2Rα/IL-15Rα colocalized with GM1 ganglioside-rich lipid rafts, but MHC I clusters retracted to smaller subsets of GM1- and IL-2Rα/IL-15Rα-rich areas upon knockdown. Our results prove that changes in expression level may significantly alter the organization and mobility of interacting membrane proteins.

Divide, Conquer, and Sense: CD8+CD28- T Cells in Perspective

Understanding the rationale for the generation of a pool of highly differentiated effector memory CD8⁺ T cells displaying a weakened capacity to scrutinize for peptides complexed with major histocompatibility class I molecules via their T cell receptor, lacking the “signal 2” CD28 receptor, and yet expressing a highly diverse array of innate receptors, from natural killer receptors, interleukin receptors, and damage-associated molecular pattern receptors, among others, is one of the most challenging issues in contemporary human immunology. The prevalence of these differentiated CD8⁺ T cells, also known as CD8⁺CD28⁻, CD8⁺KIR⁺, NK-like CD8⁺ T cells, or innate CD8⁺ T cells, in non-lymphoid organs and tissues, in peripheral blood of healthy elderly, namely centenarians, but also in stressful and chronic inflammatory conditions suggests that they are not merely end-of-the-line dysfunctional cells. These experienced CD8⁺ T cells are highly diverse and capable of sensing a variety of TCR-independent signals, which enables them to respond and fine-tune tissue homeostasis.

N-3 polyunsaturated fatty acids modulate B cell activity in pre-clinical models: Implications for the immune response to infections

B cell antigen presentation, cytokine production, and antibody production are targets of pharmacological intervention in inflammatory and infectious diseases. Here we review recent pre-clinical evidence demonstrating that pharmacologically relevant levels of n-3 polyunsaturated fatty acids (PUFA) derived from marine fish oils influence key aspects of B cell function through multiple mechanisms. N-3 PUFAs modestly diminish B cell mediated stimulation of classically defined naïve CD4(+) Th1 cells through the major histocompatibility complex (MHC) class II pathway. This is consistent with existing data showing that n-3 PUFAs suppress the activation of Th1/Th17 cells through direct effects on helper T cells and indirect effects on antigen presenting cells. Mechanistically, n-3 PUFAs lower antigen presentation and T cell signaling by disrupting the formation of lipid microdomains within the immunological synapse. We then review data to show that n-3 PUFAs boost B cell activation and antibody production in the absence and presence of antigen stimulation. This has potential benefits for several clinical populations such as the aged and obese that have poor humoral immunity. The mode of action by which n-3 PUFA boost B cell activation and antibody production remains unclear, but may involve Th2 cytokines, enhanced production of specialized proresolving lipid mediators, and targeting of protein lateral organization in lipid microdomains. Finally, we highlight evidence to show that different n-3 PUFAs are not biologically equivalent, which has implications for the development of future interventions to target B cell activity.

Identification and characterization of a novel splice variant of rhesus macaque MHC IA

Major histocompatibility complex class I (MHC I) molecules play a pivotal role in the immune recognition to intracellular pathogens. A number of important splice variants have already been characterized for these molecules in different species, suggesting their important roles in modulation of immune responses. In this study, we have identified and characterized a novel alternatively spliced form of rhesus macaque MHC IA (designated MHC IA-sv2) that lacks exons coding for the α2 and α3 domains. Despite lacking the α2 and α3 domains, MHC IA-sv2 is targeted to the cell surface, as a 23-kDa glycoprotein that is totally susceptible to endoglycosidase-H digestion and is reduced to 18kDa after deglycosylation with PNGase F. In contrast, the full-length MHC IA reaches the cell surface as a 43-kDa protein of form with complex-type N-glycosylation (endoglycosidase-H resistant). Moreover, we provide evidence here that MHC IA-sv2 can self-associate, forming homodimers, or associate with the fully mature MHC IA molecule, forming a heterodimeric structure in mammalian cells. These data demonstrate that the formation of heterodimers may have some functional implications in the fine tuning of MHC IA-mediated innate and adaptive immune responses.

Membrane raft organization is more sensitive to disruption by (n-3) PUFA than nonraft organization in EL4 and B cells

Model membrane and cellular detergent extraction studies show (n-3) PUFA predominately incorporate into nonrafts; thus, we hypothesized (n-3) PUFA could disrupt nonraft organization. The first objective of this study was to determine whether (n-3) PUFA disrupted nonrafts of EL4 cells, an extension of our previous work in which we discovered an (n-3) PUFA diminished raft clustering. EPA or DHA treatment of EL4 cells increased plasma membrane accumulation of the nonraft probe 1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate by ~50-70% relative to a BSA control. Förster resonance energy transfer imaging showed EPA and DHA also disrupted EL4 nanometer scale nonraft organization by increasing the distance between nonraft molecules by ~25% compared with BSA. However, changes in nonrafts were due to an increase in cell size; under conditions where EPA or DHA did not increase cell size, nonraft organization was unaffected. We next translated findings on EL4 cells by testing if (n-3) PUFA administered to mice disrupted nonrafts and rafts. Imaging of B cells isolated from mice fed low- or high-fat (HF) (n-3) PUFA diets showed no change in nonraft organization compared with a control diet (CD). However, confocal microscopy revealed the HF (n-3) PUFA diet disrupted lipid raft clustering and size by ~40% relative to CD. Taken together, our data from 2 different model systems suggest (n-3) PUFA have limited effects on nonrafts. The ex vivo data, which confirm previous studies with EL4 cells, provide evidence that (n-3) PUFA consumed through the diet disrupt B cell lipid raft clustering.

Soluble donor-like MHC class I proteins induce CD4(+)CD25(+)CD8(-)FoxP3(+) cells with potential to ameliorate graft chronic injury

Conventional immunosuppressive therapies failed to prevent allograft chronic rejection. New approaches to modulate recipient immune response are needed. Donor-like MHC class I soluble proteins demonstrated therapeutic potential to suppress chronic rejection. The present study was designed to clarify the ability of MHC class I soluble proteins to induce T regulatory cells with true regulatory potential in a fully allogeneic rat cardiac transplant model. Donor-like MHC class I proteins upregulate small population of splenic CD8(-) negative CD4(+)CD25(+)FoxP3(+) positive cells. CD4(+) splenocytes after MHC therapy suppress lymphocyte proliferation against donor antigens in vitro. ACI recipients of WF hearts treated with CD4(+) cells, induced with donor-like MHC class I proteins (CD4-MHC), demonstrated stable survival of the transplanted organ (MST >120 days; n=17). Histology revealed that grafts of recipients treated with CD4-MHC had 23.6% vessels affected 100 days postgrafting. On the contrary, hearts obtained from long-term surviving hosts treated with CD4(+) cells induced with high-dose CsA (CD4-CsA) had 50-70% of affected vessels. CD4-MHC class I treated transplants were mostly CD3(-) negative, had low level of mast and FoxP3(+) cell infiltration compared to CD4-CsA treated hearts. Intragraft CD4(+) cells were close to mast cells in morphology. The same graft tissues had similar number of CD4(+) positive cells and mast cells suggesting existence of CD4(+) positive mast cells. On the other hand, a negligible number of FoxP3(+) positive cells in the grafts after CD4-MHC treatment supports the idea of CD4(+) positive FoxP3(+) negative mast cells population. We demonstrate that donor-like MHC class I protein therapy induces population of CD4(+)CD25(+)CD8(-)FoxP3(+) cells with potential to ameliorate development of transplant vascular disease and evoke CD4(+) positive FoxP3 negative mast cells in the secondary hosts.

Effect of carbamylated erythropoietin on major histocompatibility complex expression and neural differentiation of human neural stem cells

The expression of major histocompatibility complex (MHC) on human neural stem cells (hNSCs) is tightly related to the fate of these cells in transplantation, therefore strategies to relieve rejection and promote graft survival are necessary to be applied. This study investigated the effect of carbamylated erythropoietin (CEPO) on MHC expression and differentiation of hNSCs with or without IFN-gamma incubation. Results showed that low levels of MHC molecules were expressed on hNSCs and increased by IFN-gamma. CEPO enhanced MHC-I antigens in both proliferative and differentiated hNSCs, but decreased MHC-II antigens in differentiated hNSCs and those cells exposed to IFN-gamma. Furthermore, CEPO promoted neural differentiation of hNSCs and outgrowth of neurites. Western blot analysis revealed activation of Stat3, Stat5 and Akt during these processes. These results suggest that CEPO may have immunoregulatory function in hNSCs besides its neuroprotection.

Intracellular assembly and trafficking of MHC class I molecules

The presentation of antigenic peptides by class I molecules of the major histocompatibility complex begins in the endoplasmic reticulum (ER) where the co-ordinated action of molecular chaperones, folding enzymes and class I-specific factors ensures that class I molecules are loaded with high-affinity peptide ligands that will survive prolonged display at the cell surface. Once assembled, class I molecules are released from the quality-control machinery of the ER for export to the plasma membrane where they undergo dynamic endocytic cycling and turnover. We review recent progress in our understanding of class I assembly, anterograde transport and endocytosis and highlight some of the events targeted by viruses as a means to evade detection by cytotoxic T cells and natural killer cells.