The immune system plays a vital role in the pathogenesis and progression of cSCC (16). Indeed, the major risk factors for cSCC development include genetically defined skin type, chronic UV exposure, chronic skin damage, and immunosuppression (17–21). The impact of immunosuppression on cSCC development has been studied most thoroughly in the context of solid organ transplant recipients. While immunosuppression is necessary to prevent transplant rejections, lifelong use of these agents has been shown to promote carcinogenesis, with cSCC being one of the most common in these patients (22).

The prominence of cSCC development in patients with iatrogenic immunosuppression strongly suggests that cSCC may have an inherent ability – that other cancers lack – to circumvent cancer immune surveillance. It has been shown in a large series of renal transplant patients that immunosuppression increases cSCC formation up to 250-fold in comparison to immunocompetent patients (23–25). The degree of immunosuppression may correspond to cSCC incidence, where reduction of immunosuppression reduces the total number, and rate of formation of cSCC (26). Numerous immunosuppressive drugs have been linked to cSCC development, namely calcineurin inhibitors (26, 27), glucocorticoids (28–30), and biologics (infliximab (31, 32), etanercept (32–34), adalimumab (32)). Furthermore, when solid organ transplant recipients do develop cSCC, their tumors tend to be more aggressive and carry a higher risk for metastasis (35). Indeed, iatrogenic immunosuppression via calcineurin inhibitors inhibit Langerhan’s cells (36, 37), dermal dendritic cells (38, 39), and T-cell signaling and proliferation, and cyclosporine directly promotes tumor development (40–42). Calcineurin inhibitors effectively disrupt IL-2 production, and as such are able to dampen immune response to allogenic antigens – a desired effect when trying to persevere tolerance towards solid organ transplants; however, this iatrogenic immunosuppression comes at a price, and significantly impairs cancer immunosurveillance (23–27). Unlike calcineurin inhibitors, the mammalian target of rapamycin (mTOR) inhibitors block IL-2 induced signal transduction, and does not abrogate IL-2 production completely, thereby allowing some functions of IL-2 to remain intact (43). As such, recently mTOR inhibitors, which include sirolimus (rapamycin), temsirolimus, and everolimus, have garnered favour because they have a lower association with de novo skin malignancies, and may in-and-of themselves have a direct anti-tumor effect. In retrospective analyses renal transplant recipients who received either sirolimus or everolimus without cyclosporine had a reduced number of de novo skin malignancies (44), and some patients experienced regression of skin cancers such as Kaposi’s sarcoma (KS) and SCC that were present prior to initiation of mTOR therapy (45–49). For these reasons, in our high-risk patient presented above, recurrent cSCC was the major factor in deciding to switch him from tacrolimus to sirolimus early on. However, he unfortunately continued to present with recurrent cSCC.

The substantially higher incidence of cSCC in immunosuppressed patients underscores the impact of the immune system in cSCC susceptibility and pathogenesis. Indeed, variations in immunological makeup may influence the ability of human hosts to recruit adaptive immune responses needed to prevent cSCC development (50). Class I and class II HLA genes encode major histocompatibility complex (MHC) proteins which allow for presentation of antigenic peptides such as tumor antigens to CD8+ and CD4+ T-cell lymphocytes, respectively. Variations in MHC proteins have been implicated in multiple cancers by influencing host defenses against tumorigenesis (50). Aberrant expression of both class I and class II HLA proteins on the surface of cSCC cancer cells is reported in both immunocompetent and immunosuppressed patients (51–55). Abnormalities in class I and class II HLA proteins are well documented in cSCC cells, reinforcing the notion that cSCC pathogenesis is inherently connected to faulty immune regulation. Several clinical studies have indicated that specific class I HLA germlines may predispose the development of cSCC in immunosuppressed patients (56–58). It has also been proposed that aberrant expression of class II HLA proteins on cSCC cancer cells may facilitate tumor escape from host defense mechanisms, as seen in other cancers including HNSCC and acute myeloid leukemia (59, 60). Furthermore, cSCC has the ability to downregulate the presentation of highly immunogenic neoantigens to TCRs (61). Thus, an important facet in the mechanism of cSCC immune escape is HLA and neoantigen dysregulation; targeting mechanisms that improve tumor-associated antigen presentation may thus be useful in the immunotherapy of cSCC.

Another avenue through which cSCC mediates immune escape is through local cytokine dysregulation. For example, cSCC significantly downregulates CCL27, a chemokine that promotes T cell homing to skin, throughout its progression from AK to malignant cSCC (62). cSCC tumors that tend to be deeper and more advanced also significantly upregulate CXCR7, which signals through CXCL12 to promote ERK signaling, thus prolonging tumor cell survival (63). Cytokine profiling of tumors reveals that in the progression to malignant cSCC, precancerous lesions dramatically upregulate production of IL-6 (64), a proinflammatory cytokine that has previously been shown to augment cSCC growth through modulation of pro-tumorigenic cytokines and angiogenic factors (65). Therefore, therapies that modulate the local cytokine and chemokine profile of cSCC may be of benefit.

Aside from surgical methods, there is a paucity of treatment modalities for aggressive cSCC. Systemic therapies for cSCC have shown limited success, although rigorous assessment of systemic therapies has been limited (66). To date, a number of systemic therapies have been used to treat cSCC, including: chemotherapeutics (cisplatin (67–69), 5-fluorouracil [5-FU] (67, 68, 70, 71), bleomycin (67), and doxorubicin (69)), 13-cis-retinoic acid (13cRA (72)), immunotherapies (interferon-α2a [IFN-α] (72)), gefitinib (73) and cetuximab (74) (agents targeting epidermal growth factor [EGFR]), and more recently nivolumab (75) and cemiplimab (76) (PD-1 immune checkpoint inhibitors).

Although chemotherapeutics have enjoyed modest success in treating surgically unresectable, and metastatic cSCC, they are accompanied by a wide range of – sometimes intolerable – gastrointestinal, hematologic, and metabolic side effects (67–69). Studies using 13cRA and IFN-α to treat SCC are conflicting; in the only trial using these compounds as adjuvant therapies in cSCC, they were ineffective (66, 72). In recent years targeting EGFR has shown some promise; indeed, EGFR is implicated in a variety of cancers including non-small cell lung cancer, colorectal cancer, pancreatic cancer, and head and neck squamous cell carcinoma (HNSCC) (77–81). Insofar as treating cSCC, two candidates have recently made it through phase II clinical trials: cetuximab, a humanized monoclonal antibody targeting EGFR, and gefitinib, which inhibits ATP-binding to EGFR (73, 74). Both compounds showed modest complete response rates, albeit with moderate toxicity.

The moderate to severe toxicities of systemic agents used to treat aggressive cSCC makes it challenging to use these drugs in the context of high-risk patients with significant comorbidities and chronic conditions, such as SOTRs. Additionally, the use of any systemic immune modifying agents may trigger immune-related adverse events (irAEs) (82) that could manifest as life-threatening (i.e. organ rejection) in patients who are iatrogenically immunosuppressed; thus, they are generally not recommended for use in this clinical context (83). Therefore, it is imperative that therapies that minimize systemic toxicities while maximizing the local tumor clearance be studied further.

The serious toxicity profile associated with systemic therapies may be altogether avoided by treating instead with intra-lesional injections. Prior studies have revealed lowered rates of toxicity associated with intra-lesional injections when compared with systemic administration. Furthermore, by delivering an increased concentration of the active agent at the site of action, increased rates of efficacy are observed (84, 85). To date, only a handful of intra-lesional agents have been used for treatment of cSCC, although this method of delivery has been extensively studied in melanoma wherein systemic adverse events are minimized and the local immune response is maximized (86).

Several case reports and small trials have been reported where actinic keratosis (AK) has been treated successfully using intra-lesional therapies (87, 88). Furthermore, 5-FU (70), methotrexate (MTX) (89), several INFs (90–93), and bleomycin (94) have all shown some utility in treating both AK and cSCC, although the data for cSCC is somewhat limited. The vast majority of reports of intra-lesional treatments of cSCC’s are case reports; however, most show good response rates and limited side effects, with the majority of side effects reported including erythema, pain and swelling at the site of injection, and occasionally, mild fever and chills. Indeed, when reflecting on our patient presented above, he experienced no immune-related adverse events. Remarkably, in a patient who cannot tolerate T cell activation (due to risk of graft failure), local injection of the potent T cell activator IL-2 was sufficient to maximize the anti-tumor immune response and did not cause any further toxicities. The patients’ side effects were limited to pain and swelling at the injection site with a short period of chills following injection, further demonstrating the benefit of intra-lesional immune therapies compared to systemic therapies.

Topical therapies that are applied locally can also mitigate the risk of systemic adverse events. A number have achieved moderate success in the treatment of AK, such as topical 5-FU, imiquimod, ingenol mebutate, and diclofenac, which are all FDA approved for this indication (95). With regards to cSCC, topical 5-FU is also commonly used (96). Additionally, early randomized controlled trials show a high degree of clinical benefit from topical imiquimod in the context of cSCC, with approximately a 70% complete response rate (97, 98). Of particular importance, imiquimod is a potent Toll-like receptor 7 (TLR7) agonist that induces local cytokine changes to cause a shift in the immunological balance intratumorally (99).

These therapies individually or in combination therefore represent a possible treatment modality in the context of high risk cSCC. In solid organ transplant recipient patients who cannot tolerate other therapies, or have failed standard local treatment with surgery and/or radiation, use of intra-lesional and/or local therapies may provide substantial clinical benefit. As iatrogenic immunosuppression plays a role in mediating immune escape of these patients’ tumors, identifying local therapies that can counteract that process is paramount.

As briefly discussed above, one of the avenues through which cSCC mediates immune escape is by downregulation of cytotoxic T cells. Thus, local therapies that promote T cell proliferation and activity are of particular interest. Outside of case reports, there are very few studies examining the immunological response to intra-lesional therapies in cSCC. Neoadjuvant intra-lesional MTX is able to induce lymphocytic inflammatory infiltrate, although the cell types and their role within the infiltrate are unclear (100). IFNα-2β;, another immunological treatment that was used intra-lesionally in a number of early studies achieved moderate clinical success, with a 88.2% complete response rate, but the specific mechanism of action is still unclear (101). However, extrapolating from its function in other contexts, it is likely upregulating a T cell-mediated anti-tumor response through the JAK1/STAT1 pathway (102). Unfortunately, in the years since these early studies, IFNα-2β; has fallen out of clinical investigation.

Interestingly, other potent T cell activating immunotherapies such as IL-2, have not yet been studied in the context of cSCC, to our knowledge. This is despite its extensive investigation as an intra-lesional agent in melanoma (86, 103–105) and HNSCC (106–109), where it mediates a shift to CD8+ T cell-mediated tumor clearance. The excellent response demonstrated by this case warrants further investigation into the possible role intra-lesional IL-2 may have in the treatment of cSCC.

Imiquimod, the other local agent used in this case, applied topically to cSCC is able to induce numerous changes targeted at augmenting T cell effector function. First, imiquimod causes dense CD8+ T cell infiltration into treated tumors, which produce significantly higher amounts of IFNγ, perforin, and granzyme compared to untreated tumors (99). In addition, treatment with imiquimod causes a shift to a polarized Th1 cytokine response (110). Production of IL-10 and TGF-β, which are known cytokines responsible for cSCC immune evasion (111), were significantly downregulated following imiquimod treatment. Imiquimod also antagonizes cSCC-mediated vascular remodeling, by upregulating E-selectin to promote cytotoxic T cell homing to the tumor (112). Furthermore, it significantly decreases FOXP3+ Treg cell levels in treated tumors, and also inhibits their function (112). Most importantly, imiquimod-treated tumors exhibit clonally expanded CD8+ T cell repertoires (112), suggesting a specific anti-tumor immune response and possibly adaptive immunity.