Sialic acid is an essential component of the spermatozoa glycocalyx and is involved in functions of spermatozoa including motility, migration and interaction with cumulus-oocyte complex (COC) (1). Sialic acids that bind to the end of glycans on the surface of cells and secrete glycoconjugates to form sialoglycans are ligands for sialic acid-binding immunoglobulin-like lectins (Siglecs) (2, 3).
Siglecs are immunoglobulin-type transmembrane proteins and comprise; a) V-set domain (sialic acid-binding N-terminal), b) C2-set domain (variable numbers of Ig domains), c) a transmembrane region and d) a cytosolic tail (1). Siglecs are commonly present on immune cells and often mediate cell-to-cell interactions and signaling (4–6). They transmit inhibitory or activating signals based on possessive ITIM or ITAM on the cytosolic tail (4–6). Siglecs bind to their ligands expressed on other cells (in trans) in order to communicate with neighboring cells and also interact with ligands expressed on the same cell (in cis) (7).
There are two primary subsets of Siglecs based on their sequence similarities and evolutionary conservation; a) conserved Siglecs, including Siglecs 1, 2, 4 and 15 and b) rapidly evolved Siglecs, including Siglec-3 (CD33) in humans and Siglecs 5, 6, 7, 8, 9, 10, 11, 14 and 16 (3). One of the main roles of some of these receptors is the fertilization process (1, 8, 9). Studies have shown the presence of Siglecs 1, 2, 5, 6, 10 and 14 on human spermatozoa (1).
To the best of our knowledge, the expression of CD33 on spermatozoa has not yet been studied. CD33 preferentially binds to α2-6- and α2-3-sialylated glycans on the surface of normal and leukemic cells (7, 10). CD33 is mostly expressed on myeloid cells and on some lymphoid cells, such as NK cells (7). Studies have shown that CD33 is an inhibitory receptor and therefore has a role in immune regulation (11). We discovered that CD33 expresses on spermatozoa following inadvertently pouring anti-CD33 antibody (instead of the intended antibody) during an experiment on a semen sample. Therefore, this study seeks to address CD33 expression on the surface of human spermatozoa. The results of this study can be beneficial regarding the use of spermatozoa as a model to study the biological function of CD33 molecule.