Generation and subcellular distribution of histamine in human blood monocytes and monocyte subsets

Objective and Design: This study was designed to establish the sites of formation and storage of histamine and histidine decarboxylase (HDC) in human monocytes and two of their subsets.¶Materials and Methods: The experiments were carried out using monocytes from buffy coats of healthy blood donors. Histamine was quantitated by RIA, HDC activity by the formation of histamine.¶Results: The monocyte subtype RM3/1 contained significantly more histamine than the subset 27E10 (0.041 ± 0.025 vs. 0.005 ± 0.004 pg/cell, p < 0.05) and also more HDC activity and HDC mRNA. After fractionation of monocyte homogenates in a discontinuous Percoll gradient or by differential centrifugation more than 80% of both, HDC activity and histamine, were recovered from the cytosolic fractions. About 50% of this histamine was found to be bound to proteins.¶Conclusions: In monocytes histamine and HDC are colocalized in the cytoplasm indicating a subcellular distribution different from mast cells or basophils. The data also show that histamine is synthesized by the monocytes themselves.