In a lethal West Nile virus (WNV) model, central nervous system infection triggered a threefold increase in CD45 int /CD11b + /CD11c ؊ microglia at days 6 -7 postinfection (p.i.). Few microglia were proliferating, suggesting that the increased numbers were derived from a migratory precursor cell. Depletion of " circulating " (Gr1 ؊ (Ly6C lo )CX3CR1 + ) and " infl ammatory " (Gr1 hi /Ly6C hi /CCR2 + ) classical monocytes during infection abrogated the increase in microglia. C57BL/6 chimeras reconstituted with cFMS -enhanced green fl uorescent protein (EGFP) bone marrow (BM) showed large numbers of peripherally derived (GFP + ) microglia expressing GR1 + (Ly6C + ) at day 7 p.i., suggesting that the infl ammatory monocyte is a microglial precursor. This was confi rmed by adoptive transfer of labeled BM (Ly6C hi /CD115 + ) or circulating infl ammatory monocytes that traffi cked to the WNV-infected brain and expressed a microglial phenotype. CCL2 is a chemokine that is highly expressed during WNV infection and important in infl ammatory monocyte traffi cking. Neutralization of CCL2 not only reduced the number of GFP + microglia in the brain during WNV infection but prolonged the life of infected animals. Therefore, CCL2-dependent infl ammatory monocyte migration is critical for increases in microglia during WNV infection and may also play a pathogenic role during WNV encephalitis.

With the recent emergence of the flavivirus, West Nile virus (WNV), in particular, the New York strain of Lineage I WNV in North America in 1999, there has been a significant increase in activity in neurotropic flavivirus research. These viruses cause encephalitis that can result in permanent neurological sequelae or death. Attempts to develop vaccines have made progress, but have been variably successful, despite considerable commercial underwriting. Thus, the discovery of ways and means to combat disease is no less urgent. As such, most recent work has been directed towards dissecting and understanding the pathogenesis of disease, as a way of informing possible approaches to abrogation or amelioration of illness. Whether inherent to flaviviruses or because humans are incidental, dead-end hosts, it is clear that these viruses interact with their human hosts in extremely complex ways. This occurs from the cellular level, at which infection must be established to produce disease, to its interaction with the adaptive immune response, which may result in its eradication, with or without immunopathological and consequent neurological sequelae. As human proximity to and contact with flavivirus insect vectors and amplifying hosts cannot practically be eliminated, our understanding of the pathogenesis of flavivirus-induced diseases, especially with regard to possible targets for treatment, is imperative.

Infiltration of Ly6Chi monocytes from the blood is a hallmark of viral encephalitis. In mice with lethal encephalitis caused by West Nile virus (WNV), an emerging neurotropic flavivirus, inhibition of Ly6Chi monocyte trafficking into the brain by anti-very late antigen (VLA)-4 integrin antibody blockade at the time of first weight loss and leukocyte influx resulted in long-term survival of up to 60% of infected mice, with subsequent sterilizing immunity. This treatment had no effect on viral titers but appeared to be due to inhibition of Ly6Chi macrophage immigration. Although macrophages isolated from the infected brain induced WNV-specific CD4+ T-cell proliferation, T cells did not directly contribute to pathology, but are likely to be important in viral control, as antibody-mediated T-cell depletion could not reproduce the therapeutic benefit of anti-VLA-4. Instead, 70% of infiltrating inflammatory monocyte-derived macrophages were found to be making nitric oxide (NO). Furthermore, aminoguanidine-mediated inhibition of induced NO synthase activity in infiltrating macrophages significantly prolonged survival, indicating involvement of NO in the immunopathology. These data show for the first time the therapeutic effects of temporally targeting pathogenic NO-producing macrophages during neurotropic viral encephalitis.

Contact hypersensitivity is a T-cell-mediated response to a hapten. Exposing C57BL/6 mice to UV B radiation systemically suppresses both primary and secondary contact hypersensitivity responses. The effects of UVB on in vivo T-cell responses during UVB-induced immunosuppression are unknown. We show here that UVB exposure, before contact sensitization, inhibits the expansion of effector CD4؉ and CD8 ؉ T cells in skin-draining lymph nodes and reduces the number of CD4؉ and IFN-␥ ؉ CD8 ؉ T cells infiltrating challenged ear skin. In the absence of UVB, at 10 weeks after initial hapten exposure, the ear skin of sensitized mice was infiltrated by dermal effector memory CD8 ؉ T cells at the site of challenge. However, if mice were previously exposed to UVB, this cell population was absent, suggesting an impaired development of peripheral memory T cells. This finding occurred in the absence of UVB-induced regulatory CD4 ؉ T cells and did not involve prostaglandin E 2 , suggesting that the importance of these two factors in mediating or initiating UVB-induced immunosuppression is dependent on UVB dose. Together these data indicate that in vivo T-cell responses are prone to immunoregulation by UVB, including a novel effect on both the activated T-cell pool size and the development of memory T cells in peripheral compartments.

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Both the UVB and UVA wavebands within sunlight are immunosuppressive. This article reviews the relationship between wavebands and dose in UV-induced immunosuppression mainly concentrating on responses in humans. It also contrasts the effects of UVB and UVA on cellular changes involved in immunosuppression. Over physiological sunlight doses to which humans can be exposed during routine daily living or recreational pursuits, both UVA and UVB suppress immunity. While there is a linear dose relationship with UVB commencing at doses less than half of what is required to cause sunburn, UVA has a bellshaped dose response over the range to which humans can be realistically exposed. At doses too low for either waveband to be suppressive, interactions between UVA and UVB augment each other, enabling immunosuppression to occur. At doses beyond where UVA is immunosuppressive, it still contributes to sunlightinduced immunosuppression via this interaction with UVB. While there is little research comparing the mechanisms by which UVB, UVA and their interactions can cause immunosuppression, it is likely that different chromophores and early molecular events are involved. There is evidence that both wavebands disrupt antigen presentation and effect T cell responses. Different individuals are likely to have different immunomodulatory responses to sunlight.

Exposure to UVB radiation before antigen delivery at an unirradiated site inhibits functional immunological responses. Mice treated dorsally with suberythemal lowdose UVB and immunized with ova in abdominal skin generated ova-specific CD8 T cells with a significantly decreased activation, expansion, and cytotoxic activity compared with unirradiated mice. UVB also impaired the delayed-type hypersensitivity (DTH) reaction to ova. Transfer of CD4 ؉ CD25 ؉ cells from UVB-exposed mice did not suppress the ova-specific CD8 T-cell response or DTH reaction in unexposed mice, confirming that systemic low-dose UVB does not induce long-lived functional regulatory CD4 ؉ CD25 ؉ T cells. Repairing cyclobutane pyrimidine dimer-type DNA damage and blocking aryl hydrocarbon receptor signaling also did not reverse the immunosuppressive effect of UVB on ova-specific CD8 T cells and DTH, suggesting that cyclobutane pyrimidine dimers and the aryl hydrocarbon receptor are not required in systemic low-dose UVBinduced immunosuppression. The known UVB chromophore, cis-urocanic acid, and reactive oxygen species triggered the inhibition of DTH caused by UVB, but they were not involved in the modulation of CD8 T cells. These findings indicate that systemic low-dose UVB impedes the primary response of antigen-specific CD8 T cells by a novel mechanism that is independent of pathways known to be involved in systemic suppression of DTH. UVB radiation (290 to 320 nm) in natural sunlight is a potent immunosuppressant. UVB can inhibit the immune system from generating optimal responses to tumors, contact haptens, and various microbial antigens (Ags; viral, fungal, and parasitic) that can lead to exacerbated disease. Alternatively, immunosuppressive UVB can also be beneficial to control autoimmune diseases, such as psoriasis 1 and experimental autoimmune encephalomyelitis. 2 The epidermis of skin absorbs UVB through chromophores, including nuclear DNA, cytoplasmic tryptophan, and extracellular trans-urocanic acid (UCA). The molecular processes that follow trigger a cascade of events that cumulates in the phenomenon of UVB-induced immunosuppression. Some of the hallmarks of this immunosuppression include UVB-induced genetic mutations in skin cells, circulation of cis-UCA from the isomerization of trans-UCA, production of reactive oxygen species (ROS), and generation of regulatory T and B cells that can transfer suppression into UVB-naïve mice. 3 Because UVB has both detrimental and beneficial effects on the immune system, it is critical to understand the mechanisms regulated by UVB so that effective prophylactic and palliative therapies can be designed for skin diseases, such as skin cancer, and immune-mediated diseases in internal organs, such as multiple sclerosis.A previous study 4 showed that UVB can inhibit CD8 and CD4 T-cell responses to haptens. In a model of contact hypersensitivity (CHS), we demonstrated that a low dose of UVB (approximately 5 minutes of summer sunlight in Sydney, Australia, at midday) is sufficient to inhibit the activati...

Brm is an ATPase subunit of the SWI/SNF chromatin-remodelling complex. Previously, we identified a novel hotspot mutation in Brm in human skin cancer, which is caused by exposure to ultraviolet radiation (UVR). As SWI/SNF is involved in DNA repair, we investigated whether Brm-/- mice had enhanced photocarcinogenesis. P53+/- and Brm-/-p53+/- mice were also examined as the p53 tumor suppressor gene is mutated early during human skin carcinogenesis. Mice were exposed to a low-dose irradiation protocol that caused few skin tumors in wild-type mice. Brm-/- mice with both p53 alleles intact had an increased incidence of skin and ocular tumors compared to Brm+/+p53+/+ controls. Brm loss in p53+/- mice did not further enhance skin or ocular cancer incidence beyond the increased photocarcinogenesis in p53+/- mice. However, the skin tumors that arose early in Brm-/- p53+/- mice had a higher growth rate. Brm-/- did not prevent UVR-induced apoptotic sunburn cell formation, which is a protective response. Unexpectedly, Brm-/- inhibited UVR-induced immunosuppression, which would be predicted to reduce rather than enhance photocarcinogenesis. In conclusion, the absence of Brm increased skin and ocular photocarcinogenesis. Even when one allele of p53 is lost, Brm has additional tumor suppressing capability.