537 Tumour-homing T cells in the blood predict responsiveness to anti-PD1 immunotherapy in metastatic melanoma patients

<h3>Background</h3> Immune checkpoint blockade is routinely used to treat melanoma. Yet while some patients experience complete and long-lasting tumour regression, others respond poorly or not at all. For therapies targeting the PD1 pathway, a pre-existing anti-tumour T cell response appears essential, which is ‘reinvigorated’ following treatment.^{1 2} But until now, there has been no simple way to measure the quality of this pre-existing T cell response in patients. Here, we hypothesise that T cells responsible for effective anti-tumour immunity in melanoma patients can be detected and quantified in blood using a combination of markers of antigen responsiveness and tumour-homing capacity. <h3>Methods</h3> We collected pre-treatment blood samples from metastatic melanoma patients (n = 46) prior to receiving their first cycle of anti-PD1 therapy and analysed T cells using flow cytometry. In parallel, patient responses to treatment were monitored via radiologic imaging. Tumour biopsies from a subset of patients were used in multispectral immunofluorescence imaging to characterise intra-tumour T cells and blood vessels. We obtained fresh tumour tissue and blood from a separate cohort of melanoma patients with brain metastases and compared T cells from blood and tumour using high-parameter flow cytometry, single cell transcriptomics and TCR sequencing. <h3>Results</h3> Co-expression of the proliferation marker Ki67 and the peripheral homing marker CD15s (sialyl Lewis X) identified a rare subset of blood CD8 T cells that strongly associated with radiologic response and overall survival following anti-PD1 therapy. These CD15s⁺ Ki67⁺ CD8 T cells displayed enhanced binding to E-selectin, an adhesion molecule expressed on vascular endothelium that captures CD15s-expressing T cells from the bloodstream to promote tissue entry.^{3 4} Moreover, this subset was enriched in progenitor exhausted T cells, which are known to contribute to the anti-PD1 response.^{2 5} Detailed analysis of patient-matched blood and tumour tissue samples demonstrated that the frequency of CD15s⁺ Ki67⁺ CD8 T cells in blood correlated with CD8 T cell density in matched tumours, and TCR sequencing revealed a clonal relationship between these circulating and tumour-infiltrating T cells, providing direct evidence of tumour-homing activity. <h3>Conclusions</h3> Our observations reveal a tractable approach to accurately predict anti-PD1 therapy responsiveness and guide personalised treatment decisions. Furthermore, our study suggests a critical role for the CD15s/E-selectin axis in determining the effectiveness of PD1 blockade. <h3>References</h3> Huang AC, Postow MA, Orlowski RJ, Mick R, Bengsch B, Manne S,<i> et al</i>. T-cell invigoration to tumour burden ratio associated with anti-PD-1 response. <i>Nature</i> 2017;<b>545</b>:60–5. Gebhardt T, Park SL, Parish IA. Stem-like exhausted and memory CD8(+) T cells in cancer.<i>Nat Rev Cancer</i> 2023;<b>23</b>:780–98. Ohmori K, Kanda K, Mitsuoka C, Kanamori A, Kurata-Miura K, Sasaki K,<i> et al</i>. P- and E-selectins recognize sialyl 6-sulfo Lewis X, the recently identified L-selectin ligand. <i>Biochem Biophys Res Commun</i> 2000;<b>278</b>:90–6. Hobbs SJ, Nolz JC. Regulation of T Cell Trafficking by Enzymatic Synthesis of O-Glycans. <i>Frontiers in Immunology</i>. 2017;<b>8</b>:600. Connolly KA, Kuchroo M, Venkat A, Khatun A, Wang J, William I, <i>et al</i>. A reservoir of stem-like CD8(+) T cells in the tumor-draining lymph node preserves the ongoing antitumor immune response.<i> Sci Immunol</i> 2021;<b>6</b>:eabg7836. <h3>Ethics Approval</h3> This study was approved by the Central Adelaide Local Health Network Human Research Ethics Committee (approval number HREC/16/RAH/95).