Abstract
Lymphatic vessels play a critical role in mounting a proper immune response by trafficking peripheral immune cells to draining lymph nodes. Mast cells (MCs) are well known for their roles in type I hypersensitivity reactions, but little is known about their secretory regulation in the lymphatic niche. MCs, as innate sensor and effector cells, reside close to mesenteric lymphatic vessels (MLVs), and their activation and ability to release histamine influences the lymphatic microenvironment in a histamine-NF-κB-dependent manner. Using an established experimental protocol involving surgical isolation of rat mesenteric tissue segments, including MLVs and surrounding perilymphatic tissues, we tested the hypothesis that perilymphatic mesenteric MCs possess histamine receptors (HRs) that bind and respond to the histamine released from these same MCs. Under various experimental conditions, including inflammatory stimulation by LPS, we measured histamine in mesenteric perilymphatic tissues, evaluated expression of histidine decarboxylase in MCs along with the degree of MC degranulation, assessed the functional status of HRs in MCs, and evaluated the ability of histamine itself to induce MC activation. Finally, we evaluated the importance of MCs and HR1 and -2 for MLV-directed trafficking of CD11b/c-positive cells during acute tissue inflammation. Our data indicate the existence of a functionally potent MC-histamine autocrine regulatory loop, the elements of which are crucially important for acute inflammation-induced trafficking of the CD11b/c-positive cells toward MLVs. This MC-histamine loop serves as a first-line cellular servo control system, playing a key role in the innate and adaptive immune response as well as NF-κB-mediated maintenance of body homeostasis.