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e23574 Background: FDCS and HS are rare tumors arising from sustentacular cells of the lymph node, rather than lymphocytes. Surgical resection is often employed in case of localized disease, but for unresectable disease, anthracycline-based therapy is commonly used. PD-L1 staining is reportedly positive in 50-80% of FDCS. As a result, we treated 3 patients (pts) with ICI ipilimumab and nivolumab (Ipi/Nivo), and demonstrated the activity of these agents in these rare cancers. Methods: Two FDCS pts and one HS pt, all with B symptoms at the start of treatment, received Ipi/Nivo in the approved dose/schedule (Table). The HS patient was HIV(+) with undetectable viral load on treatment. One FDCS patient received radiation therapy to an active site of disease during immunotherapy. We reimaged patients every 2 months for the first 6 months, then less frequently. PDL-1 IHC and mutation data were available in all cases. Results: Mutation burden varied from low to intermediate; mutations in kinase genes were also observed. Case 1 (FDCS) demonstrated RECIST 1.1 complete response, but also received radiation, and Case 2 (FDCS) had RECIST partial response. Both continue on treatment 9 months after initiation. Case 3 (HS) demonstrated a minor response to treatment, then disease worsened after 4 months, and he subsequently received radiation to the residual site of disease. All patients showed resolution of B symptoms on ICI. Conclusions: We treated these pts with ICI based on their role as antigen presenting cells, as well as their high PD-L1 expression. We speculate that FDCS and related tumors such as HS represent extreme examples of cancers that have evidence of tertiary lymphoid structures, increasingly recognized as important in ICI responsiveness. Prospective clinical trials of ICI in these diagnoses will help us understand the basis of immune responses to ICI in other cancers. [Table: see text]