Authors: Vincent C Lombardi, Svetlana F Khaiboullina, Kenny L De Meirleir, Tanja Mijatovic, Jan Hulstaer
Publication: The Journal of Immunology (2016)

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a heterogeneous illness characterized by a number of comorbid conditions; gastrointestinal (GI) dysregulation make up one subgroup of this disease. IgA is the most abundant antibody isotype found in mucosal secretions including the gut. In a process of class switch recombination (CSR), that relies on the interaction of plasmacytoid dendritic cells (pDCs) with B cells, in a T cell independent (TI) manner, low-affinity IgA are produced that limit the adhesion of commensal bacteria to intestinal epithelia without neutralizing them. These low-affinity antibodies also limit bacterial overgrowth and potential bacterial translocation thus maintaining gut homeostasis. This process is known as “immune exclusion”. Two ligands on the surface of pDCs that are obligatory for the process; the membrane bound form of APRIL and BAFF. The upregulation of APRIL and BAFF on the surface of pDCs is dependent on low-level expression of type I interferon (IFN) which is produced by intestinal stromal cells in response to Toll-like receptor (TLR) engagement. Previous studies suggest that peripheral pDCs are significantly lower in subjects with ME/CFS when compared to controls and studies conducted by us further suggest these cells likely redistribute from the periphery to the gut. We have observed that, in contrast to controls, gut-associated pDCs in subjects with ME/CFS lack APRIL and BAFF expression. These data support a model of gut pathology in ME/CFS whereby dysregulated pDCs fail to promote the production of low-affinity IgA through the process of TI activation of B cells, thereby leading to bacterial overgrowth, dysbiosis, bacterial translocation and systemic immune activation.