Myeloid Cell Expression of LACC1 is Required for Bacterial Clearance and Control of Intestinal Inflammation

Background & aims: Loss of function variants in the laccase domain containing 1 gene (LACC1) are associated with immune-mediated diseases, including inflammatory bowel diseases. It is not clear how LACC1 balances defenses against intestinal bacteria vs intestinal inflammation or what cells are responsible for this balance in humans or mice.

Methods: Lacc1^-/- mice and mice with myeloid specific disruption of Lacc1 (Lacc1^Δmye) were given oral Salmonella Typhimurium or dextran sodium sulfate. CD45RB^hiCD4⁺T cells were transferred to Lacc1^-/-Rag2^-/- mice to induce colitis. Organs were collected and analyzed by histology and for mRNA and protein expression. Bone marrow-derived macrophages and dendritic cells, lamina propria macrophages and mesenteric lymph node dendritic cells were examined. We performed assays to measure intestinal permeability, cell subsets, bacterial uptake and clearance, reactive oxygen species, nitrite production, autophagy, signaling, and cytokine levels.

Results: Lacc1^-/- mice developed more severe T cell-transfer colitis than wild-type mice and had an increased burden of bacteria in intestinal lymphoid organs, which expressed lower levels of T-helper 1 (Th1) and Th17 cytokines and higher levels of Th2 cytokines. Intestinal lymphoid organs from mice with deletion of LACC1 had an increased burden of bacteria after oral administration of S. Typhimurium and following administration of dextran sodium sulfate compared with wild-type mice. In macrophages, expression of LACC1 was required for toll like receptor-induced uptake of bacteria, which required PDK1, and MAPK- and nuclear factor κB-dependent induction of reactive oxygen species, reactive nitrogen species, and autophagy. Expression of LACC1 by dendritic cells was required for increased expression of Th1 and Th17 cytokines and reduced expression of Th2 cytokines upon co-culture with CD4⁺ T cells. Mice with LACC1-deficient myeloid cells had an increased burden of bacteria and altered T-cell cytokines in intestinal lymphoid organs, similar to Lacc1^-/- mice. Complementation of cytokines produced by myeloid cells to co-cultures of LACC1-deficient myeloid and wild-type CD4⁺ T cells restored T cell cytokine regulation. When S. Typhimurium-infected Lacc1^Δmye mice were injected with these myeloid-derived cytokines, intestinal tissues increased production Th1 and Th17 cytokines and bacteria were reduced.

Conclusions: Disruption of Lacc1 in mice increases the burden of bacteria in intestinal lymphoid organs and intestinal inflammation following induction of chronic colitis. LACC1 expression by myeloid cells in mice is required to clear bacteria and to regulate adaptive T-cell responses against microbes.

Keywords: Crohn’s disease; genetics; innate immunity; signaling.