Macrophage-Restricted Interleukin-10 Receptor Deficiency, but Not IL-10 Deficiency, Causes Severe Spontaneous Colitis (1)
Most tissue macrophages are established prebirth and subsequently main- tain themselves through longevity and limited self-renewal. Intestinal macrophages rely in contrast on the constant replenishment by blood monocytes. Local macrophage differentiation and conditioning thus have emerged as a key element for the maintenance of gut homeostasis. Newly arriving Ly6C+ blood monocytes are in the healthy mouse intestine conditioned to acquire a noninflammatory gene-expression profile and differentiate into chemokine receptor CX3CR1hi macrophages.
In the gut, IL-10 is produced by T cells, B cells, and macrophages, as well as certain nonhematopoietic cells, usually after activating stimuli. IL-10-deficient mice develop spontaneous enterocolitis. The specific roles of IL-10 production and IL- 10 sensing by intestinal CX3CR1hi macrophages in gut homeostasis were reported.
Cx3cr1creIl10rafl/fl males exhibited growth retardation compared to their Il10rafl/fl littermates, with reduced body mass and about 20 percent reduction in average weight at the age of 6 months, similar to IL-10-deficient animals. However, female Cx3cr1creIl10rafl/fl mice were less affected, with a milder reduc- tion in body mass and about 60 percent incidence of rectal prolapse. Histological assessment revealed chronic inflammation characteristics only in Cx3cr1creIl10rafl/fl mice with normal appearance of their Il10rafl/fl littermates. Multiplex cytokine analysis of sera and supernatants of colon explant cultures disclosed a significant increase in IL-6, IL-17, and IL-10 in Cx3cr1creIl10rafl/fl mice.
Microarray analysis revealed 156 upregulated and 176 downregulated genes that had significantly changed at least 2-fold in IL-10Ra-deficient versus WT macrophages. The genes upregulated in Il10ra/ macrophages revealed a marked proinflammatory signature similar to the profile of macrophages isolated from Il10/ mice, including elevation of Trem-1, Nos2, IL-23a, Ccl5, Clec9A, Ccr7, and Saa3 mRNA. qPCR analysis confirmed the differential expression of these genes. The balance of PGE2 synthesis by cyclooxygenases (COX1 and COX2) and PGE2 degradation by 15-hydroxyprostaglandin dehydrogenase (Hpgd). Cx3cr1creIl10rafl/fl macrophages displayed reduced expression of Hpgd mRNA, a finding confirmed by qPCR. The cells furthermore exhibited a reduction of arachidonate 5-lipoxygenase (Alox5) and leukotriene C4 synthase (Ltc4s) expression, i.e., two enzymes required for leukotriene synthesis.
GWASs revealed strong association of the allelic variants of the IL10 and IL10R genes with IBD, and in particular UC, suggesting impaired IL-10 signaling as key afflicted pathway in the development of human intestinal inflammation. Dysbiosis caused by antibiotica exposure and Salmonella infection, triggers expression of CCR7 by CX3CR1hi cells and their migration toward mLN. The spontaneous colitis of IL-10-deficient animals and most likely also that observed in cx3cr1creIl10rafl/fl model are driven by the commensal gut microbiota. Thus, a Myd88 deficiency in macrophages can restore homeostasis in Il10/ mice. Helicobacter hepaticus-bearing WT mice, but not animals lacking these bacteria, respond to antibody-mediated IL10R neutralization by colitis development.
A link between IL-10 and PGE2 has been previously noted in studies involving cultured human macrophages. Moreover, also the IL-10-dependent bacterial clearance in a meningitis model is associated with the potential of this cytokine to suppress PGE2 and inflammatory monocytes were shown to directly inhibit neutrophil activation in a PGE2-dependent manner.