Quantitative Trait Locus Analysis Implicates CD4⁺/CD44high Memory T Cells in the Pathogenesis of Murine Autoimmune Pancreatitis

The mouse strain MRL/MpJ is prone to spontaneously develop autoimmune pancreatitis (AIP). To elucidate the genetic control towards the development of the phenotype and to characterize contributions of immunocompetent cell types, MRL/MpJ mice were interbred with three additional strains (BXD2/TYJ, NZM2410/J, CAST/EIJ) for four generations in an advanced intercross line. Cellular phenotypes were determined by flow cytometric quantification of splenic leukocytes and complemented by the histopathological evaluation of pancreatic lesions. An Illumina SNP array was used for genotyping. QTL analyses were performed with the R implementation of HAPPY. Out of 41 leukocyte subpopulations (B cells, T cells and dendritic cells), only three were significantly associated with AIP: While CD4⁺/CD44^high memory T cells and CD4⁺/CD69⁺ T helper (Th) cells correlated positively with the disease, the cytotoxic T cell phenotype CD8⁺/CD44^low showed a negative correlation. A QTL for AIP on chromosome 2 overlapped with QTLs for CD4⁺/CD44^high and CD8⁺/CD44^high memory T cells, FoxP3⁺/CD4⁺ and FoxP3⁺/CD8⁺ regulatory T cells (Tregs), and CD8⁺/CD69⁺ cytotoxic T cells. On chromosome 6, overlapping QTLs for AIP and CD4⁺/IL17⁺ Th17 cells and again FoxP3⁺/CD8⁺ Tregs were observed. In conclusion, CD4⁺/CD44^high memory T cells are the only leukocyte subtype that could be linked to AIP both by correlation studies and from observed overlapping QTL. The potential role of this cell type in the pathogenesis of AIP warrants further investigations.

Identification of quantitative trait loci for murine autoimmune pancreatitis

BACKGROUND AND AIMS

Autoimmune pancreatitis (AIP) represents a rare but clinically relevant cause of pancreatic inflammation. Using MRL/Mp mice as a model of spontaneous AIP, the genetic basis of the disease was studied.

METHODS

To identify quantitative trait loci (QTL) of AIP, an advanced intercross line was studied, originating from MRL/MpJ parental mice and the following three mouse strains: Cast (healthy controls), BXD2 (susceptible to collagen induced arthritis), and NZM (a model of lupus erythematosus). This concept was chosen to identify both general autoimmune disease associated loci and AIP specific QTL. Therefore, generation G4 of outbred intercross mice was characterised phenotypically by scoring histopathological changes of the pancreas and genotyped with single nucleotide polymorphism (SNP) arrays. Data were analysed with the R implementation of HAPPY.

RESULTS

Five QTLs, correlating with the severity of AIP, were identified. Two of them mapped to chromosome 4 and one to chromosomes 2, 5, and 6, respectively. The QTL on chromosome 6 displays the highest LOD score (5.4) and contains the C-type lectin domain family 4 member a2 in its peak region, which encodes a receptor protein of dendritic cells that has previously been implicated in autoimmune diseases such as Sjogren's syndrome. AIP candidate genes of other QTL's include heterogeneous nuclear ribonucleoprotein A3; nuclear factor, erythroid derived 2, like 2; Sjogren syndrome antigen B; and ubiquitin protein ligase E3 component n-recognin 3.

CONCLUSIONS

This study has identified QTLs and putative candidate genes of murine AIP. Their functional role and relevance to human AIP will be studied further.