The Role of Lymphotoxin Signaling on Commensal Bacteria-Induced Inflammation

Mice deficient in LTα1β2 or LTβR develop splenomegaly in a microbiota-dependent manner. This was observed following the treatment of antibiotics and using germfree (GF) mice. The derivation of GF Ltbr-/- mice resulted in the absence of splenomegaly. Furthermore, this work also suggests the presence of specific gut bacteria that causes splenomegaly in an LT-dependent manner. In addition, previous work has demonstrated the importance of ILC3 in mediating mucosal immunity and maintaining mucosal barrier homeostasis in the intestinal tissue. Through the use of various LTβ and LTβR conditional knockout mice, we were able to demonstrate that retinoic acid-related orphan receptor γT (RORγt) cells expressing LTα1β2, and providing active LT signaling, prevents the development of splenomegaly. Specifically, ILC3s expressing LTα1β2 is both necessary and sufficient to prevent microbial-induced inflammation. Going further, previous work had characterized the importance of the gut microbiota In processing carbohydrates for nutrient absorption and demonstrated that in the absence of the gut microbiota, GF mice fed a high fat diet (HFD) were resistant to weight gain. This work demonstrates that the complexity of the carbohydrates used in HFD experiments in GF mice alter weight gain, not necessarily the presence or absence of the microbiota. Specifically, we characterized that GF mice fed a diet rich in maltodextrin were resistant to DIO compared to GF mice fed a diet rich in sucrose. Thus we have contributed to the current understanding of how the gut microbiota promotes DIO. Finally, we sought to understand whether the increased basal inflammation in specific pathogen-free (SPF) Ltbr-/- mice prevented the mice from gaining weight on HFD. We determined that a HFD alters the colonic tissue and decreases colonic tissue density in a diet-dependent manner. We demonstrated that in addition to GF Ltbr-/- mice not developing splenomegaly, but GF Ltbr-/- mice also were able to gain weight and developing DIO. This implies the microbiota-induced inflammation has a role in the previously observed leanness phenotype.