In addition to their well characterized role in allergic inflammation, recent data confirm that mast cells play a more extensive role in a variety of immune responses. However, their contribution to autoimmune and neurologic disease processes has not been investigated. Experimental allergic encephalomyelitis (EAE) and its human disease counterpart, multiple sclerosis, are considered to be CD4+ T cell–mediated autoimmune diseases affecting the central nervous system. Several lines of indirect evidence suggest that mast cells could also play a role in the pathogenesis of both the human and murine disease. Using a myelin oligodendrocyte glycoprotein (MOG)-induced model of acute EAE, we show that mast cell–deficient W/Wv mice exhibit significantly reduced disease incidence, delayed disease onset, and decreased mean clinical scores when compared with their wild-type congenic littermates. No differences were observed in MOG-specific T and B cell responses between the two groups, indicating that a global T or B cell defect is not present in W/Wv animals. Reconstitution of the mast cell population in W/Wv mice restores induction of early and severe disease to wild-type levels, suggesting that mast cells are critical for the full manifestation of disease. These data provide a new mechanism for immune destruction in EAE and indicate that mast cells play a broader role in neurologic inflammation.

Once considered to be of sole importance in allergy and parasitic infections, the role of mast cells in other pathologic and protective immune responses is becoming increasingly evident. We previously demonstrated that mast cells contribute to the severity of EAE, the rodent model of multiple sclerosis. Here we show that one mode of mast cell action is through effects on the autoreactive T cell response. Early indices of both peripheral CD4 and CD8 T cell activation, including IFN-c production and increases in CD44 and CD11a expression, are attenuated in mast cell-deficient (W/W v ) mice after myelin oligodendrocyte glycoprotein priming when compared to WT animals. Reduced infiltrates of activated T cells in the central nervous system are also observed. Importantly, selective repletion of the mast cell compartment restores most T cell responses in the lymph nodes and the central nervous system, correlating with reconstitution of severe disease. The adoptive transfer of WT-derived encephalitogenic T cells results in significantly less severe disease in W/W v recipients, indicating that mast cells also exert potent effects after the initial T cell response is generated. Our data provide the first in vivo evidence that mast cells can significantly influence T cell responses and suggest that mast cells exacerbate disease during both the inductive and effector phases.

Mast cell-deficient mice (W/Wv) exhibit significantly reduced severity of experimental allergic encephalomyelitis (EAE), a murine model of multiple sclerosis. In this study, the contribution of FcR-mediated mast cell activation to disease was examined. W/Wv mice were reconstituted i.v. with bone marrow-derived mast cells (BMMCs) from wild-type mice or those lacking functional FcRs. Eight weeks later, EAE was induced by immunization with the myelin oligodendrocyte glycoprotein 35–55 peptide. Disease scores were analyzed in reconstituted mice and compared with age-matched W/Wv mice and wild-type littermates. Mice reconstituted with FcRγ−/− BMMCs or FcγRIII−/− BMMCs exhibited less severe clinical symptoms similar to W/Wv controls, while reconstitution with FcRIIB−/− BMMCs resulted in disease significantly more severe than wild-type controls. Notably, mice reconstituted with FcγRIII−/− BMMC exhibit a relapsing-remitting course of disease. These data demonstrate that both activating and inhibitory FcRs expressed on mast cells influence the course of EAE.

The regulated expression of intercellular adhesion molecule-1 (ICAM-1) by cytokines such as tumor necrosis factor ␣ (TNF-␣) plays an important role in inflammation and immune responses. Induction of ICAM-1 gene transcription by TNF-␣ has previously been shown to be dependent upon a region of the ICAM-1 5-flanking sequences that contains a modified B site. We demonstrate here that this modified B site alone is insufficient for induction of transcription by TNF-␣. Site-directed mutagenesis of both the B site and specific flanking nucleotides demonstrates that both the specific 5-and 3-flanking sequences and the modified B site are necessary for TNF-␣ induction. Further, site-directed mutagenesis of this modified B site to a consensus B site allows it to mediate transcriptional activation in response to TNF-␣, even in the absence of specific flanking sequences. Transcription through this minimal ICAM-1 TNF-␣-responsive region can be driven by coexpression of p65, and the minimal response element interacts with p65 and p50 in supershift mobility shift assays. However, when in vitro transcription/translation products for the Rel proteins are used in an electrophoretic mobility shift assay, only p65 is capable of binding the minimal response element while both p50 and p65 bind a consensus B oligonucleotide. Additionally, in the absence of the specific flanking nucleotides, the ICAM-1 B site is incapable of DNA-protein complex formation in both electrophoretic mobility shift assay and UV cross-linking/SDS-polyacrylamide gel electrophoresis analysis. These results demonstrate the requirement for specific flanking sequences surrounding a B binding site for functional transcription factor binding and transactivation and TNF-␣-mediated induction of ICAM-1.

The acquisition of an IL-4-producing phenotype in Th2 cells requires IL-4 signaling through the STAT6 pathway during T cell differentiation. In this study we demonstrate that, unlike in naive T cells, IL-4 is not necessary for the development of an IL-4-producing phenotype in mast cells. Bone marrow-derived mast cell precursors from STAT6-/- mice can differentiate into mature cells that express IL-4 levels comparable to those of wild-type mast cells. In differentiated mast cells, activation in the presence of neutralizing anti-IL-4 antibodies or mutation of the consensus STAT6 sites does not diminish IL-4 promoter activity, indicating that IL-4 is not required for active transcription. Taken together, these data suggest that mast cell IL-4 production is not STAT6 dependent, providing evidence that these cells could generate IL-4 needed for the initiation and amplification of an effective Th2 immune response.