Despite the data available in the literature, the debate on a possible association between cutaneous malignancies and MS therapy with mAbs is still open.
4.1. Natalizumab
Among the mAbs used in MS, the largest available data concerns skin malignancies reported in treatment with NTZ. NTZ acts by inhibiting VLA-4, an integrin that plays an important role in melanocyte cell homeostasis. Integrins constitute a large family of heterodimeric transmembrane glycoproteins responsible for mediating cell-cell and cell-environment interactions. Integrins consist of two subunits: the α-subunit, ranging in size from 120 to 170 kDa, and the β-subunit, measuring 90–100 kDa. In humans, there are 18 α-subunits and 8 β-subunits that can combine to form 24 different integrins, each characterized by unique binding properties, tissue distribution, and biological functions. Notably, integrin β1 is the most abundantly expressed integrin subunit. It forms heterodimers with at least 12 α-subunits, giving rise to 12 different isoforms [50].
A member of this family is the very late activation antigen-4 (VLA-4, α4β1), which is expressed under physiological conditions in various leukocyte subtypes. It is also identified on melanoma, osteosarcoma, and rhabdomyosarcoma cells [51].
Several studies suggest that VLA-4 expression on melanoma cells may promote metastatic spread of the tumor: VLA-4 can interact with its ligand, vascular cell adhesion molecule-1 (VCAM-1), which is expressed by activated endothelium. This interaction mediates adhesion and facilitates subsequent transmigration of tumor cells. In contrast to benign melanocytic lesions, malignant melanomas exhibit elevated expression of VLA-4, enabling tumor cells migration through vascular system into any tissue in which endothelial VCAM-1 is expressed. Therefore, the increased expression of VLA-4 on melanoma is correlated with an unfavorable clinical outcome and, if so, inhibition of α4β1 integrin could prevent the spread of metastases [51].
Moreover, there is some evidence that the α4β1 integrin is involved in the lymph node dissemination of melanoma cells [52]. Specifically, α4β1 expressed on tumor cells mediates their binding to lymphatic endothelial cells (LECs) via VCAM-1. In addition, the lymphangiogenic growth factor VEGF-C, secreted by tumor cells and transported in the extracellular matrix, appears to promote the expression and activation of α4β1 integrin on LECs, while inhibition of α4β1 in LECs appears to significantly prevent lymphangiogenesis at the tumor periphery and the formation of lymph node metastases [52]. Many studies have pointed out that tumor lymphangiogenesis plays a role in promoting lymphatic metastases, providing a direct conduit for tumor cells to escape to nearby draining lymph nodes. In addition, overexpression of integrin α4β1 on primary melanoma cells appears to be associated with increased bone metastasis, probably through interaction with VCAM-1, which is constitutively expressed on bone marrow stromal cells [53].
While several studies suggest that VLA-4 expression may promote melanoma metastasis, Qian et al. showed that overexpression of the cell surface integrin α4β1 prevented the invasion of highly metastatic melanoma cells by facilitating homotypic intercellular adhesion, without having a significant effect on tumorigenicity and tumor cell growth [54]. Therefore, there is a concern that an antibody such as NTZ that binds to α4 may influence melanoma cell replication, invasion, and migration at the cellular level. Recent evidence suggests that the effect of NTZ may vary depending on the drug dose and the specific melanoma cell line [
49]. In particular, studding in vitro three different human melanoma cell lines (LCP-Mel, GR-Mel and WM115) derived from primary tumors, Carbone et al. found that, regarding GR-Mel and LCP-Mel, cell migration significantly augmented after treatment with NTZ. On the other hand, WM115 cell line had a lower migration rate, upon treatment with NTZ [
49]. Furthermore, NTZ could affect melanoma development and progression through different mechanisms, one of these may be interference with innate immunity.
Several studies have demonstrated the significant role of natural killer cells (NK) in the defense against melanoma [55].
NK cells, as effector lymphocytes in the innate immune system, play a crucial role in controlling diverse tumors and microbial infections through a dual mechanism. This involves both cytotoxic functions and the production of cytokines. The activation of NK cells is regulated by an elaborate receptor system, that includes several cell surface activating and inhibitory receptors. This system enables NK cells to differentiate between normal cells expressing MHC class I and diseased cells lacking MHC class I molecules on the surface, a phenomenon observed in neoplastic cells, including melanomas. VLA-4 is expressed on NKs and is involved in their migration across endothelial membranes. The observation that blockade of VLA-4 reduces NK cytotoxicity and modulates crosstalk with melanoma cells suggests that VLA-4 plays a role not only in NK cell adhesion and migration across the endothelial barrier, but also as an activating signal in the key functions of these cells. Moreover, studies have shown that prolonged in vitro treatment of human NK cells expressing integrin α4β1 with inhibitors such as NTZ, led to a decrease in NK cell degranulation as well as a reduced NK cells migration towards melanoma cells. Finally, α4β1 integrin expression was reduced by NTZ treatment both in vitro and in pwMS, and decreased with the duration of NTZ therapy, suggesting that this drug may alter NK-mediated immune surveillance against melanoma with a protumorigenic outcome [55].
Kimura et al. have shown that NTZ, as an α4 antagonist, can act on different types of T lymphocytes expressing this antigen (TH1, TH17, T-reg), but appears to exert its effect primarily on T-reg lymphocytes, reducing their circulating levels and disrupting the balance between inflammatory and regulatory T cells in the CD49d+ population [56].
T-regs play a crucial role in maintaining immune balance and preventing autoimmune reactions, yet they can hinder anti-tumor immune responses, potentially exacerbating cancer progression. Elevated T-reg numbers and a low CD8+ T cell/Treg ratio are often associated with poor prognosis in several cancer types, including melanoma, head and neck squamous cell carcinoma, ovarian cancer, and colorectal carcinoma [57].
The functional importance of T-regs in cancer has been demonstrated in murine models of melanoma, where transient depletion of T-regs leads to enhanced anti-tumor immunity, improved tumor clearance, and prolonged survival. Numerous studies have shown an increased presence of T-regs in the peripheral blood of patients with metastatic melanoma compared to healthy individuals of similar age. In addition, T-regs are highly concentrated within the tumor microenvironment of melanoma patients, including primary lesions, affected lymph nodes, and metastatic sites, where they exert potent immunosuppressive effects [58].
In this context, the anti-Treg action carried out by NTZ, may have beneficial effects in patients with melanomatous skin lesions.
4.2. Anti CD20 mAbs
The limited data available on skin malignancies in patients treated with OCR result from pivotal and open-label trials. These data allow a reflection on the role of CD20 in dermatological neoplastic lesions.
Several studies have reported significant amounts of infiltrating CD20+ cells in melanoma lesions [59,60,61,62,63]. However, although the role of B cells in melanoma has been extensively investigated, the results of studies have been conflicting, resulting in both pro- and anti-tumor functions. Some studies have shown that high percentages of both intratumoral and peritumoral B cell infiltrates correlate positively with both survival and metastasis rate [59,60]. Conversely, other studies found a correlation between poor prognosis and intra-tumoral CD20+ cell infiltration, or no significant correlation between these 2 parameters [62,63]. The conflicting results of some studies could be due to several factors, mainly the different stages of melanoma [64].
Several studies have shown that the tumor-promoting activity of sunlight in the skin is mediated by UV-induced DNA damage, suppression of anti-tumor immune responses and promotion of subcutaneous inflammation. A more recent finding is that UV suppresses immunity in mice by activating a specific subtype of regulatory B cells. Using a murine model of photo-carcinogenesis, L-F Kok et al. showed that the depletion of UV-activated immunoregulatory B cells limits UV-induced skin tumor growth, improves survival and/or prevents metastasis to skin-draining lymph nodes. The researchers investigated the effect of anti-CD20 antibodies compared to placebo in mice exposed to increasing doses of UV radiation, before and after the appearance of UV-induced skin lesions [65]. They showed no prophylactic effect of B-cell depletion before the appearance of skin lesions on UV-induced skin cancer, while treatment of tumour-bearing mice with anti-CD20 antibodies significantly reduced tumor growth and metastasis.
In line with these findings, previous studies have shown that UV-activated B cells suppress T-cell-driven immune responses, in part by interfering with dendritic cells and producing immunoregulatory IL-10 [66]. Kok et al., with their studies, ascertained the therapeutic efficacy of anti-CD20 mAbs in mice with UV-induced skin tumors, raising the possibility of using the same strategy in humans upon demonstration that the results obtained in mice hold true for humans [65].