Role of the aging on the ᵧdelta T-cells in metastatic cutaneous melanoma progression

Melanoma incidence, metastasis, and mortality are significantly associated with age. Interestingly within the clinic, melanoma incidence is low in young adults, peaks between ages 65-79, and decreases thereafter (79+). This phenomenon has never been studied as pre-clinical studies predominantly focus on young (8-week-old) mouse models. Syngeneic melanoma cells have been injected by IV (lung colonization) and hydrodynamic tail vein injection (liver colonization) into C57/Bl/6 young (8 weeks), aged (12 months) and geriatric (18-24 months) male mice. Spleen, lungs and liver have been collected to analyze metastasis and immune infiltration by flow cytometry. H&E and IHC staining have been performed to quantify the number of metastases and to determine different immune markers (e.g. CD45, CD8, CD4). Our data highlighted that middle aged mice had significantly increased ᵧdelta T cell infiltration in the metastatic lung and liver relative to young and geriatric mice, which had less metastasis. Based on this, we used a ᵧdelta T cell mouse model of depletion coupled with depletion antibodies against gamma delta in young and geriatric mice respectively. Finally, our preliminary data indicated that upon reactivation in middle-aged mice, melanoma cells secreted PROS1, which drives cancer proliferation. Its effects on the immune system within our model have not been studied. We overexpressed PROS1 in melanoma cells and injected them via IV and HDTV and analyzed metastasis and ᵧdelta T cell infiltration. Our data shows that middle-aged mice have significantly increased lung and liver metastasis relative to young mice. Interestingly, geriatric mice have lower levels of metastasis, replicating what is seen in the clinic. Assessment of immune cell infiltration confirmed that ᵧdelta T-cells were significantly reduced in middle-aged mice relative to young and geriatric mice. We hypothesized that they may play an anti-tumor role in metastasis. To study this, we injected tumor cells as a primary tumor, IV, and HDTV into a transgenic ᵧdelta TCR diphtheria toxin depletion model and found that while the primary tumor grew slower, ᵧdelta depleted mice had increased lung and liver metastasis. Use of a ᵧdelta depletion antibody in geriatric mice replicated this phenotype. Finally, IV and HDTV injection of PROS1 overexpressing melanoma cells significantly increased lung and liver metastasis while subsequently decreasing delta T cells infiltration in young mice. Age induced decrease of ᵧdelta T cell infiltration in middle-aged mice, induced largely by PROS1 secretion from melanoma cells, promotes aggressive metastases. Adoptive treatment with ᵧdelta T cells or use of a PROS1 inhibitor may be a viable therapeutic option for metastasis in elderly individuals.