The Siglec family of cell surface molecules, of which Siglec-3 (CD33) is a member, shares properties including cytoplasmic ITIM motifs. One member, Siglec-8, has two isoforms, Siglec-8 and Siglec-8L, with the latter expressing ITIMs that could be responsible for its proapoptotic effects.

Expression patterns of Siglec-3-related structures by eosinophils, basophils, and mast cells have not been compared.

We investigated baseline and activation-related expression of Siglecs −3, −5, −8 and −10 on these cells.

RT-PCR was used to detect mRNA expression. Specific antibodies recognizing Siglecs were used for flow cytometry or western blotting. For priming, eosinophils or basophils were cultured with or without cytokines for 18 hours.

mRNA for the Siglec-8L isoform was detected in all eosinophil preparations (n=6); most but not all also expressed mRNA for Siglec-8. Basophils, HMC-1, LAD-2 and cord blood-derived mast cells consistently expressed both Siglec-8 isoforms. This was confirmed by western blotting, which revealed both 45 and 75 kDa proteins. Total surface Siglec-8 and −8L, as assessed by flow cytometry, were similar in all types of eosinophil donors, and was not altered by cytokine priming with IL-5 or GM-CSF, nor with IL-3 for basophils. By flow cytometry human basophils expressed several Siglecs (Siglec-3 > Siglec-5 > Siglec-8; Siglec-10 undetectable), similar to levels expressed on HMC-1 cells. For eosinophils, surface expression was Siglec-8 > Siglec-3, with both Siglec-5 and Siglec-10 undetectable.

Many CD33-subfamily Siglecs are expressed by human eosinophils, basophils and mast cells. Further studies are needed to explore the functional consequences of these observations.