mMCP-11 is the first mast cell tryptase whose expression is documented in mouse basophils.

Tryptases and chymases are the major proteins stored and secreted by mast cells, and they have various biological functions. However, the nature of proteases produced by basophils has been poorly characterized, particularly in mice. mMCP-11 is the most recently discovered mast cell tryptase in mice and was originally identified as Prss34, which is transcribed in some mast cell-like cell lines and at the early stage in the culture of BMMC with IL-3. Curiously, Prss34 is preferentially expressed in the BM and spleen among normal tissues in contrast to other mast cell tryptases. Therefore, it remains elusive what types of cells express mMCP-11 in vivo. Here, we show that mMCP-11 is highly expressed by primary basophils and to a much lesser extent, by some mast cells. Prss34 transcripts were detected abundantly in primary and cultured basophils and very weakly in peritoneal mast cells or cultured BMMC. Conversely, transcripts for mMCP-6 and mMCP-7 tryptases were preferentially expressed by cultured and peritoneal mast cells but not basophils. We established a mMCP-11-specific mAb and showed that mMCP-11 proteins are indeed expressed by primary basophils and those infiltrating the affected tissues during allergic inflammation and parasitic infections. Some primary mast cells also expressed mMCP-11 proteins, albeit at a much lower level. Thus, basophils rather than mast cells are the major source of mMCP-11. This is the first study to demonstrate that mouse basophils produce a trypsin-like protease.