Background: Peripheral blood neutrophils of untreated human cancer patients have been shown to have normal, increased, and decreased phagocytic activity, killing capacity, and/or oxidative burst activities. Objectives: The objectives of this study were to evaluate oxidative burst and phagocytic activities of peripheral blood neutrophils from tumor‐bearing dogs before therapy and compare them with neutrophil function of healthy control dogs. Methods: Heparinized whole blood was obtained from dogs with high‐grade lymphoma (n=23), sarcoma (n=13), or carcinoma (n=11), and healthy control dogs (n=11) for flow cytometric evaluation of oxidative burst and phagocytic activities. Percentage of bursting cells and amount of oxidative burst activity were determined after stimulation with phorbol 12‐myristate 13‐acetate (PMA) or Escherichia coli. Percentage of phagocytic cells and amount of phagocytic activity were determined after incubation with fluorescent E. coli. Results: Compared with control dogs, dogs with sarcoma (P=.004) and carcinoma (P=.05) had a lower percentage of neutrophils exhibiting oxidative burst activity after stimulation with PMA. Phagocytic activity was significantly lower in dogs with sarcomas compared with control dogs (P<.0001) and dogs with lymphoma (P=.01). Conclusions: Untreated carcinomas and sarcomas in dogs may suppress the percentage of neutrophils capable of oxidative burst when stimulated by PMA. Furthermore, sarcomas also may suppress the amount of phagocytic activity per neutrophil. Until further studies can be performed, the clinical significance of these findings is unknown.

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