Infiltrating monocytes trigger EAE progression, but do not contribute to the resident microglia pool.

In multiple sclerosis and the experimental autoimmune encephalitis (EAE) mouse model, two pools of morphologically indistinguishable phagocytic cells, microglia and inflammatory macrophages, accrue from proliferating resident precursors and recruitment of blood-borne progenitors, respectively. Whether these cell types are functionally equivalent is hotly debated, but is challenging to address experimentally. Using a combination of parabiosis and myeloablation to replace circulating progenitors without affecting CNS-resident microglia, we found a strong correlation between monocyte infiltration and progression to the paralytic stage of EAE. Inhibition of chemokine receptor-dependent recruitment of monocytes to the CNS blocked EAE progression, suggesting that these infiltrating cells are essential for pathogenesis. Finally, we found that, although microglia can enter the cell cycle and return to quiescence following remission, recruited monocytes vanish, and therefore do not ultimately contribute to the resident microglial pool. In conclusion, we identified two distinct subsets of myelomonocytic cells with distinct roles in neuroinflammation and disease progression.

CD34 is required for dendritic cell trafficking and pathology in murine hypersensitivity pneumonitis.

RATIONALE: Although recent work has shown that CD34 plays an important role in the trafficking of inflammatory cells during Th2-biased inflammatory responses, its role in Th1/Th17-biased disease as well as dendritic cell (DC) trafficking is unknown. OBJECTIVES: We used CD34-deficient mice (Cd34(-/-)) to investigate the role of CD34 in the Th1/Th17-biased lung inflammatory disease, hypersensitivity pneumonitis (HP). METHODS: HP was induced in wild-type (wt) and Cd34(-/-) mice by repeated intranasal administration of Saccharopolyspora rectivirgula antigen. Lung inflammation was assessed by histology and analysis of bronchoalveolar lavage cells. Primary and secondary immune responses were evaluated by cytokine recall responses of pulmonary inflammatory cells as well as draining lymph node cells. MEASUREMENTS AND MAIN RESULTS: Cd34(-/-) mice were highly resistant to the development of HP and exhibited an inflammatory pattern more reflective of a primary response to S. rectivirgula rather than the chronic lymphocytosis that is typical of this disease. Cytokine recall responses from Cd34(-/-) lymph node cells were dampened and consistent with a failure of antigen-loaded Cd34(-/-) DCs to deliver antigen and prime T cells in the draining lymph nodes. In agreement with this interpretation, adoptive transfer of wt DCs into Cd34(-/-) mice was sufficient to restore normal sensitivity to HP. CD34 was found to be expressed by wt DCs, and Cd34(-/-) DCs exhibited an impaired ability to chemotax toward a subset of chemokines in vitro. Finally, expression of human CD34 in Cd34(-/-) mice restored normal susceptibility to HP. CONCLUSIONS: We conclude that CD34 is expressed by mucosal DCs and plays an important role in their trafficking through the lung and to the lymph nodes. Our data also suggest that CD34 may play a selective role in the efficient migration of these cells to a subset of chemokines.

Bone marrow-derived mast cells accumulate in the central nervous system during inflammation but are dispensable for experimental autoimmune encephalomyelitis pathogenesis.

Reports showing that W/W(v) mice are protected from experimental autoimmune encephalomyelitis (EAE, a murine model of multiple sclerosis), have implicated mast cells as an essential component in disease susceptibility, but the role of mast cell trafficking has not been addressed. In this study, we have used both mast cell transplantation and genetic mutations (Cd34(-/-), W/W(v), W(sh)/W(sh)) to investigate the role of mast cell trafficking in EAE in detail. We show, for the first time, that bone marrow-derived mast cells are actively recruited to the CNS during EAE. Unexpectedly, however, we found that EAE develops unabated in two independent genetic backgrounds in the complete absence of mast cells or bone marrow-derived mast cell reconstitution. We conclude that although mast cells do accumulate in the brain and CNS during demyelinating disease via peripheral mast cell trafficking, they are completely dispensable for development of disease.

Local self-renewal can sustain CNS microglia maintenance and function throughout adult life.

Microgliosis is a common response to multiple types of damage in the CNS. However, the origin of the cells involved in this process is still controversial and the relative importance of local expansion versus recruitment of microglia progenitors from the bloodstream is unclear. Here, we investigated the origin of microglia using chimeric animals obtained by parabiosis. We found no evidence of microglia progenitor recruitment from the circulation in denervation or CNS neurodegenerative disease, suggesting that maintenance and local expansion of microglia are solely dependent on the self-renewal of CNS resident cells in these models.

CCR2 regulates development of Theiler's murine encephalomyelitis virus-induced demyelinating disease.

Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease, a murine model for multiple sclerosis, involves recruitment of T cells and macrophages to the CNS after infection. We hypothesized that CCR2, the only known receptor for CCL2, would be required for TMEV-induced demyelinating disease development because of its role in macrophage recruitment. TMEV-infected SJL CCR2 knockout (KO) mice showed decreased long-term clinical disease severity and less demyelination compared with controls. Flow cytometric data indicated that macrophages (CD45(high) CD11b(+) ) in the CNS of TMEV-infected CCR2 KO mice were decreased compared with control mice throughout disease. CD4(+) and CD8(+) T cell percentages in the CNS of TMEV-infected control and CCR2 KO mice were similar over the course of disease. There were no apparent differences between CCR2 KO and control peripheral immune responses. The frequency of interferon-gamma-producing T cells in response to proteolipid protein 139-151 in the CNS was also similar during the autoimmunity stage of TMEV-induced demyelinating disease. These data suggest that CCR2 is important for development of clinical disease by regulating macrophage accumulation after TMEV infection.

CCL2 transgene expression in the central nervous system directs diffuse infiltration of CD45(high)CD11b(+) monocytes and enhanced Theiler's murine encephalomyelitis virus-induced demyelinating disease.

CCL2 is a member of the CC chemokine family that mediates the migration and recruitment of monocytes and T cells and has been identified in the central nervous system (CNS) during several neuroinflammatory diseases. In order to examine the biological effect of constitutive CCL2 expression in the CNS, the authors engineered a mouse that expressed CCL2 in the CNS under control of the human glial fibrillary acidic protein (hGFAP) promoter. The results demonstrated that transgenic expression of CCL2 in the CNS resulted in diffuse CNS monocyte infiltration and accumulation. Transgenic CCL2 expression did not alter normal development, differentiation, or function of T cells. There was no evidence of overt CNS disease or other pathologic phenotype when mice were left unchallenged with antigen or uninfected. However, when CCL2 transgenic mice were given a peripheral challenge of lipopolysaccharide (LPS), an inflammatory infiltrate with organized perivascular lesions developed. Infection of the transgenic mice with Theiler's murine encephalomyelitis virus (TMEV) resulted in accelerated onset and increased severity of clinical and histological disease. These results suggest that CCL2 expression in the CNS is a major pathogenic factor that drives macrophage accumulation in the development of CNS inflammatory disease.