Diet and autoimmunity
Great question. I'm still doing a bit of research but here is one reference - as always more research in needed (of course).
Role of “Western Diet” in Inflammatory Autoimmune Diseases
Arndt Manzel,1 Dominik N. Muller,2 David A. Hafler,3,4 Susan E. Erdman,5 Ralf A. Linker,1 andMarkus Kleinewietfeld3,4,6 is one such study that "discusses the current knowledge relative to the association of “Western diet” with autoimmunity, and highlights the role of T cells as central players linking dietary influences to autoimmune pathology." With regards to T cells the article also references that "leptin is critical in the regulation of energy balance and body weight, but additionally may co-stimulate T cell proliferation". The research concludes that this coupled with other findings that "on a molecular level, WAT-derived adipokines and the resultant systemic inflammation can highly impact T cell responses and thus potentially have a direct influence on autoimmune diseases."
This is but one observation but the Western high fat, high salt, processed foods etc. diet does align with increased obesity, metabolic syndrome, and cardiovascular disease and potentially also autoimmune diseases, although more research is required. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4034518/