Macrophages and Fibroblasts Differentially Contribute to Tattoo Stability.

BACKGROUND: Little information is available about the complexity and function of skin cells contributing to the high stability of tattoos. It has been shown that dermal macrophages play an important role in the storage and maintenance of pigment particles. By contrast, the impact of dermal fibroblasts, forming the connective tissue of the skin, on the stability of the tattoo is not known. METHOD: In this study, we compared the cell number and the particle load in dermal macrophages versus dermal fibroblasts, isolated from tail skin of tattooed mice. RESULTS: Microscopic analysis revealed that both cell populations contained the tattoo particles, although in largely different amounts. A small number of macrophages with high side scatter intensity contained a large quantity of pigment particles, whereas a high number of dermal fibroblasts harbored only a few pigment particles. Using the CD64dtr mouse model that allows for selective, diphtheria toxin-mediated depletion of macrophages, we have previously shown that macrophages hold the tattoo in place by capture-release and recapture cycles. In the tattooed skin of macrophage-depleted mice, the content of pigment particles in fibroblasts did not change; however, the total number of fibroblasts carrying particles increased. CONCLUSION: The present study demonstrates that dermal macrophages and fibroblasts contribute in different ways to the tattoo stability and further improves our knowledge on tattoo persistence.

Absence of MHC class II on cDC1 dendritic cells triggers fatal autoimmunity to a cross-presented self-antigen.

Conventional dendritic cells expressing the XCR1 chemokine receptor (cDC1s) excel at cross-presentation. Here, we developed and used a mouse model in which a Cre recombinase is expressed under the control of the Xcr1 gene while preserving XCR1 expression. We used it to generate mice with conditional deletion of MHC class II (MHCII) molecules on cDC1s. By preventing cDC1s to receive suppressive regulatory T cell inputs via MHCII-restricted interactions, the objective of the present study was to gauge whether MHCII-deficient cDC1s lose their capacity of tolerizing autoreactive CD8+ T cells. Whereas MHCII+ cDC1 readily cross-tolerized strongly autoreactive CD8+ T cells specific for a keratinocyte-derived self-antigen, MHCII-deficient cDC1s converted them into potent effectors capable of triggering a fast-onset lethal autoimmunity associated with severe skin histopathological manifestations. Preventing egress of such pathogenic self-reactive CD8+ T cell effectors from the cutaneous draining lymph nodes abrogated the autoimmune condition. Therefore, our results revealed that the cross-tolerizing capacity of cDC1s is not a property fully acquired at the time they undergo homeostatic maturation but needs to be enforced via MHCII-restricted, suppressive interactions with regulatory T cells.

Unveiling skin macrophage dynamics explains both tattoo persistence and strenuous removal.

Here we describe a new mouse model that exploits the pattern of expression of the high-affinity IgG receptor (CD64) and allows diphtheria toxin (DT)-mediated ablation of tissue-resident macrophages and monocyte-derived cells. We found that the myeloid cells of the ear skin dermis are dominated by DT-sensitive, melanin-laden cells that have been missed in previous studies and correspond to macrophages that have ingested melanosomes from neighboring melanocytes. Those cells have been referred to as melanophages in humans. We also identified melanophages in melanocytic melanoma. Benefiting of our knowledge on melanophage dynamics, we determined the identity, origin, and dynamics of the skin myeloid cells that capture and retain tattoo pigment particles. We showed that they are exclusively made of dermal macrophages. Using the possibility to delete them, we further demonstrated that tattoo pigment particles can undergo successive cycles of capture-release-recapture without any tattoo vanishing. Therefore, congruent with dermal macrophage dynamics, long-term tattoo persistence likely relies on macrophage renewal rather than on macrophage longevity.