PO-322 Investigating co-expression of the immune checkpoints heme oxygenase-1 and PD-L in breast cancer

Abstract

IntroductionUnprecedented clinical outcomes have been observed in a variety of cancers by targeting the immune checkpoint molecules PD-1/PD-L and CTLA-4. However, a significant proportion of patients fail to respond to these therapies, indicating the potential existence of further significant immune regulatory pathways. Heme Oxygenase-1 (HO-1) is an enzyme responsible for the breakdown of heme to biliverdin, iron and the immune suppressive molecule carbon monoxide. We recently demonstrated that HO-1 acts as an immune checkpoint by suppressing chemotherapy-elicited anti-tumour immune responses in murine models of breast cancer. In the present study we investigate the co-expression of HO-1 with the clinically targeted immune checkpoints PD-L1/2 to explore the immune checkpoint landscape in breast cancer.Material and methodsWe investigate the expression of checkpoint molecules PD-L1/2 and HO-1 (HMOX1) in a microarray dataset of tumour tissue from breast cancer patients. We then analyse a single-cell RNAseq dataset, grouping tumoral cells according to their expression profiles using t-distributed stochastic neighbour embedding (tSNE) followed by density clustering, and probe for the expression of genes of interest. We utilise the aggressive spontaneous murine model of mammary adenocarcinoma (MMTV-PyMT) to investigate the therapeutic efficacy of targeting PD-1 (the receptor for PD-L1/2) and/or HO-1 activity.Results and discussionsAnalysis of microarray data from patients with breast cancer demonstrated that the expression of HO-1 and PD-L1/2 were associated within the tumour microenvironment. Analysis of unbiased human breast cancer single-cell RNAseq data indicated that myeloid cells were a significant source of HO-1 and PD-L1/2 in the tumour microenvironment. The MMTV-PyMT mouse model paralleled these clinical observations where both HO-1 and PD-L1/2 were expressed by the tumour associated macrophages. Using a chemotherapy-elicited anti-tumour immune response we pharmacologically inhibited HO-1 activity and/or neutralised PD-1 receptors using blocking antibodies to evaluate the therapeutic efficacy of targeting these respective pathways. In this model, targeting HO-1 alongside chemotherapy resulted in a greater regression of tumour growth than targeting PD-1.ConclusionThese data prompt the need to further investigate the redundancy and hierarchy of immune checkpoints in the tumour microenvironment to effectively harness the anti-tumour immune response and efficiently implement immune checkpoint blockade therapy.