Immune Checkpoint Blockade Enhances Shared Neoantigen-Induced T-cell Immunity Directed against Mutated Calreticulin in Myeloproliferative Neoplasms

Abstract

Somatic frameshift mutations in the calreticulin (<i>CALR</i>) gene are key drivers of cellular transformation in myeloproliferative neoplasms (MPN). All patients carrying these mutations (<i>CALR</i> ⁺ MPN) share an identical sequence in the C-terminus of the mutated CALR protein (mut-CALR), with the potential for utility as a shared neoantigen. Here, we demonstrate that although a subset of patients with <i>CALR</i> ⁺ MPN develop specific T-cell responses against the mut-CALR C-terminus, PD-1 or CTLA4 expression abrogates the full complement of responses. Significantly, blockade of PD-1 and CLTA4 <i>ex vivo</i> by mAbs and of PD-1 <i>in vivo</i> by pembrolizumab administration restores mut-CALR-specific T-cell immunity in some patients with <i>CALR</i> ⁺ MPN. Moreover, mut-CALR elicits antigen-specific responses from both CD4⁺ and CD8⁺ T cells, confirming its broad applicability as an immunogen. Collectively, these results establish mut-CALR as a shared, MPN-specific neoantigen and inform the design of novel immunotherapies targeting mut-CALR. SIGNIFICANCE: Current treatment modalities for MPN are not effective in eliminating malignant cells. Here, we show that mutations in the <i>CALR</i> gene, which drive transformation in MPN, elicit T-cell responses that can be further enhanced by checkpoint blockade, suggesting immunotherapies could be employed to eliminate <i>CALR</i> ⁺ malignant cells in MPN.<i>This article is highlighted in the In This Issue feature, p. 1143</i>.