Immune checkpoint inhibitors (ICI) have revolutionized cancer treatment by facilitating the immune system’s ability to target malignant cells. [1, 2] Inhibition of immune checkpoints such as Programmed cell Death protein 1 (PD-1) and Cytotoxic T Lymphocyte Antigen-4 (CTLA-4) increase immune recognition in melanoma, lung cancer, and some lymphomas. [3] However, their use has been linked to the development of secondary inflammation, termed immune related adverse events (irAEs), including uveitis. [2, 4, 5]

Uveitis is a multifaceted ocular inflammatory disease, involving a complex network of molecular signaling pathways. Cytokines are pivotal mediators of inflammation and assume a central role in orchestrating the immune response within the ocular microenvironment. Infection, injury, or autoimmune/inflammatory diseases and drug-induced inflammation can lead to uveitis. Key cytokines associated in uveitis include interleukins (IL), specifically IL-6 and IL-17, tumor necrosis factor alpha (TNF-α), and interferon gamma (IFN-γ), each exhibiting distinct effects on the pathogenesis of uveitis. [6,7,8,9]

This review focuses on the role of cytokines in uveitis, with an emphasis on uveitis induced by immune checkpoint inhibitors. Understanding the cytokine profiles associated with ICI-induced uveitis not only provides insights into the mechanisms of this adverse event but also sheds light on potential unique therapeutic strategies applicable to multiple etiologies of uveitis.

Immune checkpoints are components of regulatory pathways in the immune system that help to maintain self-tolerance, averting autoimmunity. The interaction between immune checkpoint ligands on host cells and the receptors expressed on T Cells prevents the immune cell from entering an active state to mount an immune response against the host cell. [1] The PD-1 axis is an excellent example of immune cell regulation. PD-1 is a surface expressed receptor found on T cells, B cells, dendritic and NK cells. [10,11,12] Its ligand, Programmed cell death ligand 1 (PD-L1), is expressed in numerous cell types located throughout target tissues including vascular endothelial cells, mesenchymal stem cells, pancreatic islets, neurons, keratinocytes, placental tissue. Germane to this review is the presence of PD-L1 on corneal epithelial cells and retinal pigmented epithelial cells . [10, 13,14,15,16] When a T Cell major histocompatibility complex (MHC) binds to a host antigen in peripheral tissue, the presentation of PD-L1 on the host binds the PD-1 receptor, resulting in a differentiation into a regulatory T cell (Treg), a cell line responsible for promoting self-tolerance, or induction of a state of anergy (“exhaustion”) whereby it cannot be stimulated to proliferate nor mount an inflammatory response. [11, 17] Notably, some cancers are able to exploit the PD-1/PD-L1 system by expressing PD-L1 on the cell surface or even secrete soluble PD-L1 to elude the host immune system. By way of comparison, the CTLA-4 receptor-ligand interaction occurs primarily in secondary lymphoid organs, representing an earlier stage of T cell activation. [18]

Immune checkpoint inhibitor therapy has been effective in treating a growing number of malignancies by counteracting the evasion of cancers from host immune surveillance. [10,11,12] ICIs are monoclonal antibodies that directly interrupt the interaction between PD-1 and PD-L1, enabling the immune system to act against tumor cells expressing immune checkpoint ligands on the cell surface. (Fig. 1) ICI therapy is effective against a myriad of cancers and is a powerful tool against cancers that are resistant to the typical first line chemotherapies. Numerous monoclonal antibodies have been FDA approved for treatment of metastatic melanoma, small cell lung cancer, renal cell carcinoma and others. [3] Approved anti-PD-1 monoclonal antibody (mAb) therapies include pembrolizumab (Keytruda), Nivolumab (Opdivo), and Cemiplimab-rwlc (Libtayo). [3] Anti-PD-L1 mAb therapies include Atezolizumab (Tecentriq), Durvalumab (Imfinzi), and Avelumab (Bavencio). [3]

The PD-1 receptor-ligand interaction prevents T Cell activation. ICI prevent this interaction which leads to activation of T Cells. Original Image

The unfortunate but expected effect of inhibiting the immune checkpoint system is a reduction of self-tolerance and a rise in autoimmune activity by T Cells, which are clinically responsible for inflammatory and autoimmune disease states. Neuropathies, anemias, thrombocytopenia, autoimmune pancreatitis, and uveitis are immune-related adverse events (irAEs) associated with ICI therapy.

The manifestation of robust anti-self activity is a specific indication of cytotoxicity of neoplastic cell therapy. A decrease in self-tolerance, which is often detectable clinically, may provide a surrogate indicator of the efficacy of the ICI therapy and response to tumor cell recognition. Thus, the presence and level of activity of irAEs provides a proxy for the effectiveness of the antitumor therapy. Recently, studies have consistently demonstrated improved survival outcomes as patients experience an increasing number of irAEs. [1, 19, 20] The presentation of irAEs are variable, ranging from a mild dermatitis to life threatening heart failure. [19, 21] Ocular irAEs include dry eye, corneal decompensation, uveitis, ocular myasthenia, and optic neuropathy . [22,23,24] Recent literature has identified paraneoplastic forms of carcinoma associated retinopathy (CAR), melanoma associated retinopathy (MAR) and Acute Exudative Polymorphous Vitelliform Maculopathy (pAEPVM) in association with CTLA-4 and PD-1 inhibition. [25,26,27,28] Cases of pre-existing paraneoplastic retinopathies have been shown to rapidly worsen after PD-1 inhibition. [29, 30] While irAEs may be an encouraging sign for cancer treatment, they can be associated with significant morbidity and require discontinuation of therapy or treatment with systemic steroids. Though the cause of the irAE is often the blockage of a single receptor-ligand interaction (i.e., PD-1), treatment for irAEs is more complex and requires broad suppression of inflammation through systemic steroids or targeted cytokine therapy.

In particular, the etiology of ICI-Related uveitis is incompletely understood. Retinal pigment epithelial (RPE) cells natively express high levels of PD-L1, contributing to the immune privileged status of the eye and thus are an important barrier to autoimmunity. The mechanism of ICI-Related uveitis may be attributed to the reduction of self-tolerance. [15, 31] ICI related uveitis is relatively rare, occurring at a rate of 1% of patients treated on ICI over one-year. [22] Combined therapy with multiple ICIs, female gender, and metastatic melanoma may confer increased risk of uveitis. [2, 22] The uveitis is typically mild, presenting with mild to moderate anterior chamber inflammation and light sensitivity, often solely requiring corticosteroid therapy for resolution. [2, 4, 32, 33] Severe cases, however, can be vision threatening. As a specific example, the treatment of melanoma with ICI therapy can trigger a cross reactivity of normal choroidal melanocytes and malignant melanoma cells, resulting in a Vogt-Koyanagi-Harada (VKH)-like panuveitis that frequently requires the stoppage of the inciting ICI. [4] This heterologous immunity has been observed in ICI treatment of cutaneous, subcutaneous and uveal metastatic melanomas. [22, 34, 35] ICI related uveitis can be treated as idiopathic uveitis with topical, oral, intravitreal, or intravenous (IV) steroids. Though these treatments are often effective in achieving quiescence, among their numerous side effects are the acceleration of cataracts, elevated intraocular pressure, and glaucoma. [36, 37]

The mechanism underlying irAEs is a cytokine dysregulation triggered by loss of self-tolerance. Several studies have investigated the cytokine profile in patients experiencing irAEs and found that the medley of cytokines implicated varies depending on the offending cancer type. A majority of studies have been reported on patients with metastatic melanoma treated with ICI therapy. A recent study of 98 such patients, treated either with anti-PD-1 monotherapy, nivolumab or pembrolizumab, or in combination with anti-CTLA-4 therapy, ipilimumab, were assessed longitudinally for severe irAEs with cytokine bioassays. [38] In this study, 11 cytokines were elevated in metastatic melanoma patients with severe irAEs: Fractalkine, fibroblast growth factor 2 (FGF-2), interferon alpha 2 (IFN-α2), IL-12p70, IL-1a, IL-1B, IL-1RA, IL-2, and IL-13, granulocyte colony stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF). [38] Importantly, the anti-tumor efficacy of ICI therapy did not correlate with cytokine expression, suggesting that disruption of one or more of these cytokines may not impact ICI functionality. For patients treated with anti-PD-1 monotherapy, there was an association between treatment success and serum levels of IL-2, interferon gamma-induced protein 10 (IP-10, also known as C-X-C motif chemokine ligand 10 (CXCL 10)), and monocyte chemoattractant protein 4 (MCP-4, also known as chemokine ligand 13 (CCL13)). [38, 39]

ICI therapy is also widely used in small cell lung cancer. Recent studies assessing serum cytokine levels have implicated different cytokine profiles for irAEs in the setting of anti-PD-1 or anti-PD-L1 therapy for small cell lung cancer. A 2022 study found that after controlling for age, sex, pathological type and PD-L1 expression status, elevated IL-5, IFN-α, and IFN-γ were associated with a higher risk of irAEs. [40] A follow up 2023 study added to this list an array of cytokines including IL-1β, IL-2, IL-6, IL-8, IL-12, IL-17, IFN-α, IFN-γ, and TNF-α. [41] This study also found reduced clinical benefit of ICI treatment in patients who developed elevated serum IL-8. [41]

Targeting the PD-1 receptor has also shown encouraging results in the treatment of renal cell carcinoma (RCC). [42, 43] As in the previous example, the cytokine profile associated with irAEs in this cohort is somewhat unique; elevated levels of IP-10/CXCL10 is associated with the development of irAEs in patients with RCC undergoing combination or monotherapy with ICI. [44] The exact patient cohort was also tested for the pre and post treatment levels of cytokines associated with irAEs in lung cancer and melanoma, including IL-17 A, IL‐1β,IL‐6, IL‐8, monocyte chemoattractant protein 1 (MCP‐1), also known as chemokine ligand 2 (CCL2), and TNF-α. Paradoxically, no significant increases in these levels were detected. Additionally, IL-8, associated with both irAE development and reduced efficacy of ICI treatment for small cell lung cancer, was undetectable in this patient cohort with renal cell carcinoma, further confirming the difficulty in targeting cytokines for treatment of irAEs. [44]

The cytokines associated with immune-related adverse events (irAEs) vary significantly depending on the specific type of cancer being treated. This Venn diagram illustrates the distinct cytokine profiles observed across different cancer types in response to Anti-PD-1 Immune Checkpoint Inhibitors. The limited overlap between cytokine signatures underscores the unique immunological landscapes associated with each cancer

The degree of morbidity of irAEs can be quite significant, requiring changing or discontinuation of ICI therapy, there exists a need to select a target to prevent or treat irAEs in this vulnerable patient population. However, these studies highlight the idiosyncratic nature of the immune system’s response to “releasing the brakes” in the setting of malignancy, making target selection more difficult (Fig. 2).

Noninfectious uveitis is an autoimmune or immune-mediated disease. Noninfectious uveitis can be associated with underlying systemic disease, such as sarcoidosis, or present without underlying disease, such as serpiginous choroiditis. Underpinning these etiologies is the activation of the host immune system resulting in damage to the eye. The pathophysiology of noninfectious uveitis is may be akin to that seen in irAE in patients receiving ICI therapy. [5, 45] Research into the pathogenesis and treatment of noninfectious uveitis offers us some insight into the cytokine dysregulation seen in irAEs.

Studies of serum cytokine levels in patients with noninfectious uveitis have consistently identified associations with elevated TNF-α, IL-6, IFN-γ and IL-17 A. [6,7,8, 46,47,48,49,50] The list of cytokines is likely much longer, including IL-8, IL-12, G-CSF, GM-CSF, MCP-1, IP-10, TNF-α and VEGF. [9] Typical treatment for uveitis involves local or systemic corticosteroid therapy to dampen the host immune response, with escalation to immunomodulatory therapy if the widespread immunosuppression of corticosteroid therapy is prolonged. Biologics targeting individual cytokines can also give excellent control of uveitis.

IL-6 is a major player in uveitis, the presence of IL-6 receptors on retinal vascular endothelial cells suggests that elevated serum levels of this cytokine can produce significant visual morbidity. [51] The STOP-Uveitis randomized clinical trial compared two strengths of an anti-IL-6R antibody, tocilizumab, in patients with non-infectious uveitis. This therapy demonstrated improvement in incidence and severity of ocular and systemic disease in both groups. [52] Anti-IL-6R therapy has also demonstrated benefit in treating chronic or refractory non-infectious uveitis. [53] Tocilizumab, used to quell non-infectious uveitis, has also shown benefit in treating or preventing irAEs in the setting of anti-PD-1 therapy. [54,55,56] Given these encouraging results from systemic therapy, local ocular treatment with intravitreal injections of IL-6 antibody has also been explored in mouse models. [57]

In addition, IL-17 A has also been implicated in uveitis. Research concerning both infectious and non-infectious uveitis has shown that serum IL-17 A levels are markedly elevated compared to controls, suggesting its involvement in ocular inflammation. [47] Additionally, IL-17 A contributes to macular edema by damaging the blood-retinal barrier through JAK1 signaling. [58] Studies on anti-IL-17 treatments in rat models have demonstrated potential in reducing uveitic inflammation, resulting in milder symptoms, delayed onset, and faster resolution. [59] Although IL-17 blockade did not completely prevent experimental autoimmune uveoretinitis (EAU), it reduced the presence of Th17 cells and decreased inflammation markers IL-6 and TNF. [