Abstract
Cutaneous squamous cell carcinoma is a non-melanoma skin cancer that is among the most common cancers capable of metastasis. The majority of cutaneous squamous cell carcinomas (cSCCs) are easily treated by simple surgical excision, but there exists a subset of cases (approximately five percent) that will become invasive and metastatic. Currently, there are no clinically relevant biomarkers to identify potentially aggressive cSCC, and the biological mechanisms remain unclear.
Previous work by our laboratory found high levels of myeloid-derived suppressor cells (MDSCs), particularly the granulocytic subpopulation, in the circulating blood of renal transplant patients and patients with cSCC. MDSCs are a mixed group of immature immune cells (including a high proportion of immature granulocytes) that have been shown to inhibit anti-cancer immunosurveillance and hence facilitate tumour progression.
This study analysed circulating and tumour infiltrating immunoregulatory cell populations (MDSCs, neutrophils and lymphocytes) in the blood and tumour samples from patients with cSCC who were not on immunosuppressive medications (non-immunosuppressed, n = 29) and immunosuppressed patients with cSCC (n = 18). The frequencies of MDSCs in the immunosuppressed group were significantly higher compared to the non-immunosuppressed group when analysed as a whole. When we split the non-immunosuppressed patient group by tumour stage (high-stage tumours (n =8) and low-stage tumours (n = 21)), we found that frequencies of total MDSCs and granulocytic MDSCs were significantly higher in the blood of both the high-stage tumour group and the immunosuppressed group compared to the low-stage tumour group. When the tumours of these patients were analysed, increased peritumoural and intratumoural levels of CD66b positive neutrophils as well as higher ratios of CD66b positive neutrophils to CD8 positive lymphocytes were observed as tumour stage increased. Moderate to strong correlations between levels of tumour-associated neutrophils and lymphocytes and levels of circulating MDSCs, were also observed.
A clinical audit of cSCC patients treated in the Department of Plastic Surgery, Christchurch Hospital (2009 - 2011) was performed to investigate associations among immunoregulatory cell populations, high-risk tumour characteristics and survival in a larger cohort (n=168). Immunosuppressed patients (n = 39) had higher levels of circulating neutrophils and lower levels of lymphocytes in their blood compared to the non-immunosuppressed patients (n = 129). When the non-immunosuppressed patients were analysed independently, patients that had neutrophil counts of over 4.5 x 109/L, compared to those with lower neutrophil counts, had a significantly higher proportion of tumours that were greater than five millimetres in thickness (p = 0.03), a Clark level of five (p = 0.02) and had higher overall tumour stage (p = 0.04). Furthermore, a thickness of greater than five millimetres was the most significant predictor of overall survival in non-immunosuppressed patients, a characteristic that is given relatively little importance under the current staging system. None of the circulating immune cell populations investigated were associated with overall survival, however neutrophil levels in circulation showed some association with advanced tumour stage (p = 0.04).
This is the first study to investigate MDSCs, neutrophils and lymphocytes with clinical information in patients with cSCC. Although patient numbers were small in the current study, and survival and recurrence data for this cohort is beyond the scope of this thesis, this work will be ongoing. As part of this ongoing research, the mechanisms by which neutrophils and granulocytic MDSCs might contribute to tumour progression should be explored. This research will lead to a greater understanding of the underlying pathology of aggressive cSCC, and may provide more informative biomarkers to help identify patients with high-risk cSCC.