188P - Expression of the co-stimulatory checkpoint protein OX40L (TNFSF4) in the melanoma micro-environment

Background

Immune checkpoint inhibitors revolutionized treatment of both locally advanced and metastatic melanoma patients, yet some patients fail to achieve long-lasting benefit or progress on treatment, highlighting the need for new immune-modulatory agents. The co-stimulatory ligand OX40L and its receptor OX40 (also named TNFSF4/ TNFRSF4) were previously shown to elicit anti-tumor immune responses in pre-clinical cancer models. This led to several clinical trials of OX40-agonism as therapeutic interventions in cancer, which showed limited efficacy thus far. We previously analyzed RNA-sequencing data of OX40L expression in melanoma and found that low mRNA expression was associated with worse prognosis and with worse outcome following anti-PD1 treatment. These findings encouraged further studies to substantiate the role of OX40L in the melanoma microenvironment.

Methods

Multiplex immunofluorescent microscopy was used to evaluate the expression of OX40L/OX40 in FFPE surgical specimens of primary/metastatic melanoma tumors. Previously established cell-specific markers were used in parallel to identify OX40L- expressing cell phenotypes. Quantification was performed with GEN5 PRIME software.

Results

OX40L+ cells were detected in 15/16 tumors tested, in variable abundances, which correlated with the abundance of OX40+ cells and with co-localization events of OX40L+/OX40+ cells, suggesting functional OX40 ligand-receptor interactions. OX40L expression was frequently detected on Macrophages/Dentritic-cells, less on CD4+ or CD8+ T cells and rarely on melanoma cells. Unexpectedly, we now identified high prevalence of OX40L expression on Foxp3+ regulatory T cells (Treg), in addition to OX40, which was previously shown to antagonize their suppressive function.

Conclusions

Here we show that OX40L is abundantly expressed, together with OX40, on Treg within the melanoma micro-environment. These novel findings may suggest a role for OX40L in modulating Treg suppressive activity. If further proven, our observations may be important in prognostication and prediction of response in melanoma. Furthermore, they may point to new IO combinations for further research and have therapeutic implications towards ‘heating’ up tumors.

Legal entity responsible for the study

The authors.

Funding

Israeli Cancer Association (ICA).

Disclosure

All authors have declared no conflicts of interest.