Melanosomes and MHC class II antigen-processing compartments: a tinted view of intracellular trafficking and immunity

Melanocyte-keratinocyte cross-talk in vitiligo

Vitiligo is a common acquired pigmentary disorder that presents as progressive loss of melanocytes from the skin. Epidermal melanocytes and keratinocytes are in close proximity to each other, forming a functional and structural unit where keratinocytes play a pivotal role in supporting melanocyte homeostasis and melanogenesis. This intimate relationship suggests that keratinocytes might contribute to ongoing melanocyte loss and subsequent depigmentation. In fact, keratinocyte dysfunction is a documented phenomenon in vitiligo. Keratinocyte apoptosis can deprive melanocytes from growth factors including stem cell factor (SCF) and other melanogenic stimulating factors which are essential for melanocyte function. Additionally, keratinocytes control the mobility/stability phases of melanocytes via matrix metalloproteinases and basement membrane remodeling. Hence keratinocyte dysfunction may be implicated in detachment of melanocytes from the basement membrane and subsequent loss from the epidermis, also potentially interfering with repigmentation in patients with stable disease. Furthermore, keratinocytes contribute to the autoimmune insult in vitiligo. Keratinocytes express MHC II in perilesional skin and may present melanosomal antigens in the context of MHC class II after the pigmented organelles have been transferred from melanocytes. Moreover, keratinocytes secrete cytokines and chemokines including CXCL-9, CXCL-10, and IL-15 that amplify the inflammatory circuit within vitiligo skin and recruit melanocyte-specific, skin-resident memory T cells. In summary, keratinocytes can influence vitiligo development by a combination of failing to produce survival factors, limiting melanocyte adhesion in lesional skin, presenting melanocyte antigens and enhancing the recruitment of pathogenic T cells.

A central role for inducible heat-shock protein 70 in autoimmune vitiligo

Inducible heat-shock protein 70 (HSP70i) is a protein regulated by stress that protects cells from undergoing apoptosis. Such proteins are marvellously well conserved throughout evolution, which has placed them in the spotlight for helping to understand the intriguing relationship between infection and immunity. In the presence of stress proteins, dendritic cells (DCs) will sense this alarm signal and respond by recruiting immune cells of different plumage to fit the occasion. In times of stress, melanocytes will secrete antigen-bound HSP70i to act as an alarm signal in activating DCs that comes equipped with an address of origin to drive the autoimmune response in vitiligo. Here we pose that if the autoimmune response is funnelled through HSP70i, then blocking the stress protein from activating DCs can lend new treatment opportunities for vitiligo.

A current viewpoint of lymphangioleiomyomatosis supporting immunotherapeutic treatment options

Lymphangioleiomyomatosis (LAM) leads to hyperproliferation of abnormal smooth muscle cells in the lungs, associated with diffuse pulmonary parenchymal cyst formation and progressive dyspnea on exertion. The disease targets women of child-bearing age. Complications include pneumothoraces and chylous pleural effusions. Ten-year survival is estimated at 70%, and lung transplantation remains the only validated treatment. It has been observed that LAM cells express markers associated with melanocytic differentiation, including gp100 and MART-1. Other melanocytic markers have also been observed. The same proteins are targeted by T cells infiltrating melanoma tumors as well as by T cells infiltrating autoimmune vitiligo skin, and these antigens are regarded as relatively immunogenic. Consequently, vaccines have been developed for melanoma targeting these and other immunogenic melanocyte differentiation proteins. Preliminary data showing susceptibility of LAM cells to melanoma derived T cells suggest that vaccines targeting melanosomal antigens can be successful in treating LAM.

Processing of tumor antigen differentially impacts the development of helper and effector CD4+ T-cell responses

CD4(+) T cells contribute to the antitumor T-cell response as both effectors that promote tumor rejection and helpers that facilitate the activation of other antitumor effector cells, such as CD8(+) T cells. Maximal engagement of both effector and helper CD4(+) T-cell responses is a desirable attribute of cancer vaccines. We have employed the B16F10 murine melanoma model and a series of recombinant adenovirus (Ad) vaccines expressing mutant forms of the tumor antigen, dopachrome tautomerase, to investigate the relationship between antigen processing and the antitumor CD4(+) T-cell response. Our results have revealed an unexpected dichotomy in the generation of helper and effector CD4(+) T-cell responses where CD4(+) T effector responses are dependent upon protein processing and trafficking, whereas CD4(+) T helper responses are not. The results have important implications for strategies aimed at augmenting antigen immunogenicity by altering intracellular processing and localization.

The melanocortin-1 receptor gene polymorphism and association with human skin cancer

The melanocortin-1 receptor (MC1R) is a key gene involved in the regulation of melanin synthesis and encodes a G-protein coupled receptor expressed on the surface of the melanocyte in the skin and hair follicles. MC1R activation after ultraviolet radiation exposure results in the production of the dark eumelanin pigment and the tanning process in humans, providing physical protection against DNA damage. The MC1R gene is highly polymorphic in Caucasian populations with a number of MC1R variant alleles associated with red hair, fair skin, freckling, poor tanning, and increased risk of melanoma and nonmelanoma skin cancer. Variant receptors have shown alterations in biochemical function, largely due to intracellular retention or impaired G-protein coupling, but retain some signaling ability. The association of MC1R variant alleles with skin cancer risk remains after correction for pigmentation phenotype, indicating regulation of nonpigmentary pathways. Notably, MC1R activation has been linked to DNA repair and may also contribute to the regulation of immune responses.

Gamma-IFN-inducible-lysosomal thiol reductase modulates acidic proteases and HLA class II antigen processing in melanoma

HLA class II-restricted antigen (Ag) processing and presentation are important for the activation of CD4+ T cells, which are the central orchestrating cells of immune responses. The majority of melanoma cells either expresses, or can be induced to express, HLA class II proteins. Thus, they are prime targets for immune mediated elimination by class II-restricted CD4+ T cells. We have previously shown that human melanoma cells lack an important enzyme, gamma interferon-inducible lysosomal thiol-reductase (GILT), capable of perturbing immune recognition of these tumors. Here, we show that GILT expression in human melanoma cells enhances Ag processing and presentation via HLA class II molecules. We also show that GILT expression influences the generation of active forms of cysteinyl proteases, cathepsins B, L and S, as well as an aspartyl protease cathepsin D in melanoma cells. Mechanistic studies revealed that GILT does not regulate acidic cathepsins at the transcriptional level; rather it colocalizes with the cathepsins and influences HLA class II Ag processing. GILT expression in melanoma cells also elevated HLA-DM molecules, which favor epitope loading onto class II in the endolysosomal compartments, enhancing CD4+ T cell recognition. These data suggest that GILT-expressing melanoma cells could prove to be very promising for direct antigen presentation and CD4+ T cell recognition, and may have direct implications for the design of cancer vaccines.

TRPML cation channels regulate the specialized lysosomal compartment of vertebrate B-lymphocytes

B-lymphocytes possess a specialized lysosomal compartment, the regulated transformation of which has been implicated in B-cell antigen presentation. Members of the mucolipin (TRPML) family of cation channels have been implicated in regulated vesicular transport in several tissues, but a role for TRPML function in lymphocyte vesicular transport physiology has not been previously described. To address the role of TRPML proteins in lymphocyte vesicular transport, we analyzed the lysosomal compartment in cultured B-lymphocytes engineered to lack TRPML1 or after expression of N- or C-terminal GFP fusion proteins of TRPML1 or TRPML2. Consistent with previous analyses of lymphocytes derived from human patients with mutations in TRPML1, we were not able to detect abnormalities in the lysosomes of TRPML1-deficient DT40 B-lymphocytes. However, while N-terminal GFP fusions of TRPML2 localized to normal appearing lysosomes, C-terminal GFP fusions of either TRPML1 or TRPML2 acted to antagonize endogenous TRPML function, localizing to large vesicular structures, the histological properties of which were indistinguishable from the enlarged lysosomes observed in affected tissues of TRPML1-deficient humans. Endocytosed B-cell receptors were delivered to these enlarged lysosomes, demonstrating that a TRPML-dependent process is required for normal regulation of the specialized lysosome compartment of vertebrate B-lymphocytes.

Colocalization of glial fibrillary acidic protein, metallothionein, and MHC II in human, rat, NOD/SCID, and nude mouse skin keratinocytes and fibroblasts

The expression of glial fibrillary acidic protein (GFAP) by perivascular cells of many mammalian organs suggests an as yet unknown function of this intermediate filament protein in the maintenance of homeostasis and vascular permeability at the blood-tissue interface. Although a similar situation may exist at the air-tissue interface, the cellular distribution of GFAP in skin tissue has never been demonstrated. To approach this issue, we have employed immunofluorescence and Western blotting techniques to detect GFAP in skin sections of young and adult humans, normal rodents, and two types of mutant mice, as well as in rat lung sections, and in cultured human keratinocytes and fibroblasts. Colocalization with antigens known to be associated with GFAP in other tissues was also tested. Epidermal and hair follicle keratinocytes and dermal fibroblasts showed distinct staining for GFAP as well as colocalization with alpha-actin, metallothionein, and antigens of the class-II major histocompatibility complex (MHC II). GFAP was also identified in rat alveolar fibroblasts which, in common with keratinocytes, form part of the air-tissue interface. GFAP was upregulated together with MHC II in nude mice but was barely detectable in the skin of non-obese diabetic severe combined immunodeficiency mice, suggesting a possible involvement in antigen-presenting functions. The intriguing distribution of a common set of antigens both in certain cells of the integumentary system and at the blood-tissue interfaces of internal organs suggests the involvement of these proteins in universal mechanisms controlling tissue homeostasis and protection.

Melanogenesis and evidence for melanosome transport to the plasma membrane in a CD83 teleost leukocyte cell line

Visceral organs of ectothermic vertebrates harbour melanin-containing leukocytes termed melanomacrophages. These cells are thought to participate in immune reactions and free-radical trapping. In teleosts, the melanin-producing ability of melanomacrophages has hitherto not been confirmed by molecular techniques. Here, a leukocyte marker and the apparatus for melanosome production and transport were investigated in an Atlantic salmon (Salmo salar) pronephros-derived mononuclear leukocyte (SHK-1) cell line. The SHK-1 cells expressed transcripts specific for a mammalian CD83 homologue, a standard surface marker for activated or differentiated dendritic cells, and dopachrome tautomerase/tyrosinase-related protein-2, a melanocyte specific enzyme essential for melanin production. Reduction potential of melanin or its precursors was demonstrated histochemically after prolonged cultivation. Ultrastructural investigations revealed tyrosinase and acid phosphate activity in identical organelles and BSA-gold co-localized with multilamellar melanosomes after 2 h internalization. Apparently, melanosomes were transported and released through periodically occurring tubules fusing with the plasma membrane. Video monitoring revealed filopodia and macropinocytosis. These results showed that the SHK-1 cell line is capable of melanogenesis and melanosome secretion. Melanin-producing cells in teleost pronephros may represent a distinct CD83(+) leukocyte population consisting of phylogenetically relict multifunctional cells. This is the first report of a melanin-producing leukocyte cell-line.