Chicken CD300a homolog is found on B lymphocytes, various leukocytes populations and binds to phospholipids

• The chicken CD300 family comprises an activating CD300L-X1 and an inhibitory CD300L-B1.
• The monoclonal antibody (1D4) is specific for the CD300L-B1 with no crossreactivity to CD300L-X1.
• CD300L-B1 is expressed on all immature and mature B cells, monocytes, heterophils, macrophages and blood NK cells.
• CD300L-B1 but not for CD300L-X1 binds phosphatidylserine and phosphatidylethanolamine.

The chicken CD300 cluster contains three genes that encode inhibitory, activating and soluble forms. In the present study, we have generated a monoclonal antibody against the inhibitory CD300L-B1 molecule. The mab 1D4 was specific for the CD300L-B1 form and showed no crossreactivity with the related CD300L-X1. Virtually all bursal cells expressed CD300L-B1, whereas only a small positive subset was found in thymus that was identified as thymic B cell subpopulation. In peripheral tissues, CD300L-B1 was found to be expressed on lymphocyte subpopulations in blood and spleen. Double immunofluorescence analysis with B- and T-cell specific markers identified these subsets as B lymphocytes. In addition, analysis of PBMC revealed that CD300L-B1 was also present on monocytes, heterophils, blood NK cells and in vitro differentiated macrophages. We utilized a reporter cell line in order to identify potential ligands of CD300L-B1. When several phospholipids were tested, only phosphatidylserine and phosphatidylethanolamine were found to trigger strong reaction of the reporter cells. The two phospholipids elicited a response only in CD300L-B1 reporter cells, but not in CD300L-X1 reporter cells. Moreover the interaction could be blocked with the specific mab. In conclusion, we provide evidence for the expression of chicken CD300L-B1 on immature and mature B cells, monocytes, heterophils, macrophages and NK cells and identify phosphatidylserine and phosphatidylethanolamine as CD300L-B1 ligands.