Potential link between MHC-self-peptide presentation and hematopoiesis; the analysis of HLA-DR expression in CD34-positive cells and self-peptide presentation repertoires of MHC molecules associated with paroxysmal nocturnal hemoglobinuria

Abstract

The mechanisms of MHC allele associations with paroxysmal nocturnal hemoglobinuria (PNH) and its aplastic anemia subtype (AA/PNH) remain unclear. It might be dependent on MHC molecule functional properties, such as a scope and frequency of antigen sampling and presentation. For documented PNH-associated MHC alleles we analyzed current reference databases on MHC molecule-eluted peptide presentation repertoires and searched for a range of presented peptides. MHC class II expression was measured on CD34+ cells and appeared to be increased in PNH patients. Two class I alleles (HLA-A*24:02 and B*18:01) have been previously confirmed to associate with protection and increased risk of AA/PNH, respectively. Their product molecules presented immunodominant epitopes derived from proapoptotic (serine/threonine-protein phosphatase) and antiapoptotic (phospholipase D), respectively, intracellular enzymes dependent on phosphoinositide (PI) content. For total PNH and non-aplastic PNH (n/PNH) subtype-associated DRB1*15:01 and DRB1*04:01 class II molecules presentation of exceptionally broad arrays of their own peptide fragments has been found. We conclude that self antigen peptides presented with high frequency in the context of MHC molecules of increased expression may be involved in the immune recognition and the regulation of HSC in the periphery. The block in the normal plasma membrane PI production due to the PIG-A mutation can help explain the differences in the activation of intracellular regulatory pathways observed between PNH and normal HSC. This is evident in the variation in MHC association patterns and peptide presentation repertoires between these two groups of patients.

Fig. 1

HSCs in PNH patients express increased numbers of HLA-DR molecules as compared to normal controls. a An example of HSC compartment (CD34-positive cells) in nucleated peripheral blood cells in control (left) and PNH patient (right). CD34-APC-positive cells were gated for on the CD45-PerCP-Cy-5.5/SSC plots. For better visualization of relatively small CD34 positive compartment CD16 positive granulocytes were subtracted from the pictures. b An example of HLA-DR staining in total CD34-positive cells in control (left) and PNH patient (right). Co-staining in the presence of CD45-PerCP-Cy-5.5, CD34-APC, and HLA-DR-PE mAb is shown. c An example of the CD16-APC-Cy7 GPI-anchored protein staining in granulocyte fraction (PNH clone size) in PNH patient (left) and results of seven experiments as median of HLA-DR antigen bound per CD34-positive cell (ABC) ± standard error of mean (right). APC, allophycocyanin; PE, R-phycoerithrine; PerCP-Cy-5.5, peridinin–chlorophyll–protein complex-cyanin 5.5; APC-Cy7, allophycocyanin-cyanin 7; HSCs, hematopoietic stem cells; PNH, paroxysmal nocturnal hemoglobinuria; mAb, monoclonal antibodies

Fig. 2

Hypothetical pathways involved in MHC–self peptide-TCR recognition and PNH clone dominance. Both immunorecognition and apoptosis mechanisms are involved in clonal selection of PNH hematopoietic cells. Down-regulation of hematopoiesis and limited cytotoxicity are direct effects of low affinity MHC–self peptide-TCR immunorecognition. Self peptide repertoires are diversely presented by HSC-located MHC class II to T_regs and apoptotic pathways are diversely modulated by PI content in PIG-A mutant and normal HSCs. (A) A somatic mutation occurs in PIG-A gene in a hematopoietic stem cell followed by gradual PI imbalance in plasma membranes, followed by putative PLD, PI3K, and PKB/Akt activation and suppression of Foxo transcription factors. This makes the PIG-A mutant HSCs more resistant to apoptosis than normal HSCs. (B) T_reg-mediated self recognition of HSCs (including complex MHC class II-self peptide) results in the T_reg-activation, MHC unrestricted intercellular contact with Th CD4+ cells of any antigenic specificity and possibly other effector cells that are present in the bone marrow in the vicinity of activated T_regs. (C) The induction of this process is more productive if increased avidity of the recognition is the case, i.e., when more efficient presentation of certain HSC-derived self peptides occurs in the context of PNH associated than non-associated MHC class II molecules. (D) The bone marrow “cellular network” is downregulated, that results in IL-2 deprivation, and (E) the “cytokine network” shift toward IFN-γ production, leading to the “pro-apoptotic stress” and MHC class I and II up-regulation. The bone marrow turns to the picture of bone marrow insufficiency. In local bone marrow hematopoietic foci PNH cells overcome the pro-apoptotic stress by the PIG-A gene mutation, PI alteration and activation of anti-apoptotic pathway, so that (F) high and (G) low apoptosis, respectively, occurs in normal and PNH HSCs. At this stage the anti-apoptotic loop is closed between IFN-γ-producing cells and PIG-A mutant hematopoietic stem cells creating the mechanism of PNH clone domination. (H) Excessive presentation of certain HSC-derived self peptides in the context of up-regulated MHC class I can increase avidity of specific TCR of naive CD8-positive T cells close to the activation threshold and induce maturation of TCR. (J) Normal and PNH HSCs presenting line-specific peptides are attacked by these mature “post-naive” T cells and aplastic anemia picture is presented. This process is triggered by more efficient presentation of certain HSC-derived self peptides in the context of PNH associated than non-associated MHC class I. (K) Normal HSCs present normal levels of tissue-specific peptides. (L) Recognition with normal MHC–self peptide-TCR avidity is tolerant. HSC, hematopoietic stem cell; MHC, mayor histocompatibility complex; T_reg, regulatory T cell; naïve T CD8+, naive cytotoxic T cell; Th, helper T cells; NK, natural killer cells; TCR, T cell receptor; IL, interleukin; CSF, colony stimulating factor, TGFβ, transforming growth factor beta; IFN-γ, interferon gamma; TNF, tumor necrosis factor