| Summary: | Interactions between circulating tumour cells (CTCs) and platelets are thought to inhibit natural killer(NK)-cell-induced lysis. We attempted to correlate CTC numbers in men with advanced prostate cancer with platelet counts and circulating lymphocyte numbers. Sixty-one ExPeCT trial participants, divided into overweight/obese and normal weight groups on the basis of a BMI ≥ 25 or <25, were randomized to participate or not in a six-month exercise programme. Blood samples at randomization, and at three and six months, were subjected to ScreenCell filtration, circulating platelet counts were obtained, and flow cytometry was performed on a subset of samples (<i>n</i> = 29). CTC count positively correlated with absolute total lymphocyte count (r<sup>2</sup> = 0.1709, <i>p</i> = 0.0258) and NK-cell count (r<sup>2</sup> = 0.49, <i>p</i> < 0.0001). There was also a positive correlation between platelet count and CTC count (r<sup>2</sup> = 0.094, <i>p</i> = 0.0001). Correlation was also demonstrated within the overweight/obese group (<i>n</i> = 123, <i>p</i> < 0.0001), the non-exercise group (<i>n</i> = 79, <i>p</i> = 0.001) and blood draw samples lacking platelet cloaking (<i>n</i> = 128, <i>p</i> < 0.0001). By flow cytometry, blood samples from the exercise group (<i>n</i> = 15) had a higher proportion of CD3+ T-lymphocytes (<i>p</i> = 0.0003) and lower proportions of B-lymphocytes (<i>p</i> = 0.0264) and NK-cells (<i>p</i> = 0.015) than the non-exercise group (<i>n</i> = 14). These findings suggest that CTCs engage in complex interactions with the coagulation cascade and innate immune system during intravascular transit, and they present an attractive target for directed therapy at a vulnerable stage in metastasis.
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