Biological factors involved in the modulation of bacterial endotoxin-mediated inflammation in type 1 diabetes

Type 1 diabetes (T1D) is a disease characterized by the autoimmune destruction of insulin-producing pancreatic β cells. Diabetic nephropathy is a life-threatening complication of T1D, characterized by the progressive loss of kidney function. Approximately one-third of all patients with T1D develop diabetic nephropathy. Bacterial DNA and bacterial lipopolysaccharides (LPS) are two categories of bacterial remnants that are known to induce inflammation. Inflammation and elevated levels of bacterial remnants have previously been shown to be associated with the development of diabetic nephropathy, and there is evidence in mice that these factors play a causal role in disease progression. The general aim of this thesis is to identify biological factors that modulate bacterial remnant-mediated inflammation in patients with T1D. Specifically, we aimed to better understand the composition, origin and consequences of bacterial remnants in circulation in the context of T1D by (1) evaluating the presence of bacterial DNA in the sera of patients with T1D and controls (Study I) and (2) measuring LPS activity, inflammation, inflammatory potential and gut-related factors in the context of multiple high-fat meals given to patients with T1D and healthy controls (Studies II & III). Study I found a higher frequency of Pseudomonal (Pa) DNA in circulation as well as elevated anti-Pa IgA levels in patients with T1D. These Pa-specific IgA antibodies correlated with higher C-reactive protein, a marker for inflammation, suggesting that patients with T1D undergo recurrent or chronic Pseudomonal exposure and potentially explaining the chronic inflammation in patients with T1D. Contrary to our hypotheses, study I found no correlation between LPS activity and either bacterial DNA composition or antibodies against identified bacterial species. This may be attributable to differences in the half-lives and host clearance mechanisms of bacterial remnants. However, it is also possible that the entry mechanisms and points of entry for LPS and bacterial DNA are different. The identification of numerous bacteria known to be present in the oral cavity suggests that one possible point of entry is the oral cavity. Another possible point of entry for bacterial remnants is the gut. Circulating LPS was previously shown to increase after the ingestion of high-fat meals by healthy adults. Study II found a pronounced increase in serum markers of inflammation after multiple high-fat meals; however, our data suggest this inflammation was not attributable to increases in circulating LPS. Indeed, circulating LPS levels appeared to have no effect on immune cell activation or systemic inflammation. This led us to investigate factors related to intestinal homeostasis, particularly focusing on factors such as alkaline phosphatases, which might affect the potency of intestinally derived LPS. In Study III, we found a general disturbance in factors related to gut homeostasis in patients with T1D. Specifically, low levels of fecal alkaline phosphatase found in patients with T1D contributed to increased LPS potency in the intestine, which in turn boosted intestinal inflammation. These studies help shed light on the potential routes of entry for bacterial remnants and the possible mechanisms underlying the inflammatory response induced by high-fat meals.