The pathology of cutaneous lesions in MRL-Faslpr
mice and human CLE are similar (41
). Sunlight (UVB-exposure) triggers the expression of human CLE and human lupus (1
). Since CSF-1 is required for nephritis in MRL-Faslpr
mice, we analyzed the impact of CSF-1 in the pathogenesis of CLE in MRL-Faslpr
). By constructing unique mutant MRL-Faslpr
strains expressing varying levels of CSF-1 (high, intermediate, none), and using an ex-vivo
gene transfer system to increase intra-dermal CSF-1, we determined that CSF-1 is necessary for and incites CLE in MRL-Faslpr
mice. Hypothesizing that UVB exposure worsens CLE through a CSF-1-dependent mechanism, we showed that UVB-exposure increased Mø, apoptosis in the skin, and incited CLE in MRL-Faslpr
, but not in CSF-1-deficient MRL-Faslpr
mice. Probing deeper, we determined a CSF-1-dependent, Mø-mediated mechanism is responsible for CLE.
Lupus susceptibility genes are essential for CSF-1 mediated CLE. Using either “CSF-1 carrier cells” or UVB-exposure, CLE was induced in the lupus-susceptible MRL-Faslpr
mice, but not in the lupus-resistant BALB/c mice. This finding is consistent with sunlight (UVB) triggering CLE in lupus patients, but not in normal individuals (42
). Moreover, UVB-exposure incited a far greater accumulation of Mø within the skin of MRL-Faslpr
as compared to lupus-resistant BALB/c mice, despite inducing similar levels of CSF-1 in the skin. This finding is consistent with hyper-proliferation of Mø to CSF-1 in MRL-Faslpr
, but not in non-autoimmune strains (43
). Thus, Mø responsiveness to CSF-1 may be a critical feature of lupus susceptibility leading to CLE.
Our findings highlight a central role for Mø in CLE. These are consistent with the essential contribution of Mø to psoriasis (an inflammatory skin condition traditionally thought to be triggered by T lymphocytes) in two distinct models, a T cell dependent and T cell independent (14
). In this autoimmune skin disorder, the recruitment and activation of Mø in the psoriatic skin is a key pathogenic event in the development and maintenance of psoriatic lesions. Thus, it is tempting to suggest that the underlying events that lead to Mø recruitment, activation and the “misbehavior” of these Mø in psoriasis are linked to CSF-1. Of note, we are aware that Mø are somewhat less prevalent in human CLE than in the MRL-Faslpr
mice. We view the greater number of Mø in MRL-Faslpr
CLE as an advantage to more clearly dissect the role of these CSF-1R-bearing leukocytes in the disease process.
Our findings indicate that a rise in CSF-1 alone is not sufficient for CLE, but rather require the MRL-Faslpr
background genes. While our data suggests that Mø are central to UVB incited CLE, the increase in Mø is followed by a substantial rise in CD4 T cells. Thus, distinct populations of T cell may regulate CLE. In this regard, gross skin lesions are suppressed in MRL-Faslpr
mice lacking CD4, but not CD8 T cells (44
). On the other hand, expanding natural killer T cells (NKT) by provision of α-Galactosylceramide (45
) suppresses dermitis, and eliminating CD1d, an antigen-presenting molecule know to activate NKT cells exacerbates dermitis (46
). In contrast, CLE is accelerated in beta(2
)-microglobulin deficient MRL-Faslpr
mice in which normal development of CD1-dependent NKT cells is prevented (47
). Thus, it is not clear whether NKT cells are central to CLE. With this in mind, we plan to pinpoint the distinct T cell populations in lupus-susceptible MRL-Faslpr
mice, but not lupus-resistant strains required for UVB incited CLE.
We report the novel finding that UVB induces CSF-1 in keratinocytes, and dermal fibroblasts. We detected a rise in CSF-1 mRNA (real-time PCR) and/or protein (ELISA) following UVB-exposure in mouse keratinocytes (primary cells and PAM 212 line), primary human keratinocytes, primary mouse dermal fibroblasts and human dermal fibroblasts. In contrast, another report, using Northern analysis and proliferation assays, indicated that UVB decreases CSF-1 expression by PAM 212 mouse keratinocytes and fibroblasts of a mouse dermal line (39
). The discrepancy between these findings and ours maybe related to the differences between primary cells and cell lines and/or the assay techniques used. We are confident of our findings since our in vitro
studies are consistent with our and other’s in vivo
evidence that UVB increases intra-dermal CSF-1 (48
), and the reproducibility of our findings in two primary cell types, keratinocytes and dermal fibroblasts, derived from two species, mouse and humans.
UVB-exposure induces apoptosis in the skin and apoptotic cells have been implicated in the pathogenesis of lupus (49
). There are several mechanisms that may be instrumental in inducing apoptosis in skin parenchymal cells. For example, UVB induces direct, ligand-independent activation of membrane death receptors such as Fas/Fas-ligand up-regulation (52
). However, since MRL-Faslpr
mice lack a functional Fas antigen, we can dismiss this mechanism. Of note, the release of TNF-α and subsequent ligation of the TNF receptor (p55) mediates keratinocyte apoptosis (53
). In this regard, TNF-a, along with nitric oxide and reactive oxygen species, are mediators released by activated Mø that induce apoptosis of renal parenchymal cells, TEC (16
). In the present study, the UVB induction of CSF-1 in the skin (keratinocytes and dermal fibroblasts) recruits and activates Mø. These Mø home to sites rich in CSF-1 and release mediators that induce apoptosis in adjacent cells. This suggests that the CSF-1-dependent influx of Mø is the culprit responsible for UVB-incited apoptosis in the skin. This is consistent with the enhanced numbers of intra-dermal apoptotic cells and Mø following UVB exposure in MRL-Faslpr
, as compared to BALB/c and CSF-1-deficient MRL-Faslpr
mice. We wish to point out that MRL-Faslpr
mice are defective in phagocytosis of apoptotic cells, and that this inefficient clearance, which is also discussed in human lupus (55
) may contribute, in part, to the accumulation of intra-dermal apoptotic cells (56
). Finally, evidence indicates that apoptotic cells may be instrumental in inducing auto-Abs that promote systemic lupus (3
) Thus, it is intriguing to speculate that UVB induction of CSF-1 in the skin may initiate systemic lupus by increasing apoptotic cells in the skin, and/or causing a rise in systemic CSF-1 and/or boosting the activated Mø in the skin that may egress and home to other tissues. Future experiments will test these possibilities.
In conclusion, our studies identify a UVB-incited, CSF-1-dependent, Mø-mediated sequence leading to CLE in lupus-susceptible strains. We envision CSF-1 as the “match” and lupus susceptibility genes as the “tinder” required for CLE.