Microcalorimetry does not predict the cellular phagocytosis of latex microspheres

Current literature highlights the potential suitability of microcalorimetry for the investigation of cell-drug interactions. Previous work using bacteria or antigens derived from infectious organisms yielded conclusions that heat production is a quantitative means of measuring phagocytosis. In this study we evaluated the potential of flow-through microcalorimetry as a method of quantifying the phagocytosis of microsphere particulates. The technique avoids the need to incorporate radioactive or fluorescent markers into the particulate formulation, and would be widely applicable in biopharmaceutical research. Using the monocyte cell line Mono Mac 6 a power output of 9.00 microW per million cells was increased significantly on addition of zymosan, lipopolysaccaride (LPS) and phorbol myristate acetate but not following exposure to FITC labelled latex microspheres (LM). TNFalpha production increased on exposure to zymosan, LPS and LPS-phorbol myristate acetate, though not on exposure to LB. An assay was developed which allowed the quantification of internalised particulates in phagocytic cells using fluorescent activated cell sorting (FACS). In contrast to the microcalorimetric and TNFalpha data FACS revealed that 20% of the MM6 population phagocytosed a mean of 1.35 LM. Microcalorimetry and measurements of TNFalpha production are assays of cellular activation a phenomenon not necessarily associated with phagocytosis. FACS, however, serves as a specific and quantitative measure of phagocytosis. Microcalorimetry may not be a suitable technique for the quantitative assessment of the phagocytosis of drug delivery particulates.