Background <p>Immune checkpoint inhibitors (ICI), a type of cancer immunotherapy, can cause side effects including inflammatory arthritis (ICI-IA). Previous studies of ICI-IA do not include a thorough characterization of associated immune responses to provide potential targets for treatment. We aimed to identify cytokines uniquely increased in ICI-IA and determine correlations with IA severity and persistence.</p> Methods <p>We evaluated patients diagnosed with ICI-IA by a rheumatologist (<i>n</i> = 80); control serum was obtained from ICI-treated cancer patients without any diagnosed irAEs (<i>n</i> = 17) or diagnosed with an unrelated irAE (<i>n</i> = 19). Serum was assayed to quantify 9 cytokine levels (IFN-γ, IL-4, IL-6, IL-10, IL-12p70, IL-1α, TNF-α, IL-17a, VEGF-A) using MSD U-PLEX assay. Mann-Whitney U tests were performed to evaluate differences in cytokine levels between control and ICI-IA groups. The Kruskal-Wallis test and multivariable ordinal logistic regression were used to determine difference in cytokine levels between patients of differing disease activity.</p> Results <p>VEGF-A and TNFα were significantly elevated in patients with ICI-IA compared to ICI-controls; results persisted when restricting analyses to patients not treated with immunosuppressants at the time of sampling. ICI-IA patients were stratified by IA severity using CDAI score; there was significantly higher VEGF-A in those with higher disease activity. Ordinal logistic regression showed higher levels of IL-6 and VEGF-A were associated with higher disease activity.</p> Conclusion <p>Elevated levels of VEGF-A and TNFα are associated with ICI-IA. There was also higher IL-6 and VEGF-A among those with higher disease activity when controlling for confounding. These cytokines could be used as biomarkers of ICI-IA severity and present therapeutic targets.</p>