Annexin V binds to positively selected B cells.

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• Recombinant annexin V (rAnV) typically identifies apoptotic cells by binding externalized phosphatidylserine.
• A large fraction of B cells in murine bone marrow (BM) bind rAnV but are not apoptotic.

Purpose of the Study:

• To investigate the unexpected binding of rAnV to viable B cells.
• To determine the relationship between rAnV binding and B cell development and selection.

Main Methods:

• Flow cytometry using fluorescently labeled rAnV on murine bone marrow cells.
• Coculture of BM B cells with BM stromal cell lines.

Main Results:

• rAnV binding in developing B cells correlates with B cell receptor (BCR)-dependent selection events in B-1 and B-2 subsets.
• Nearly all B-1 B cells and splenic marginal zone B cells bind rAnV, indicating phosphatidylserine externalization upon BCR-mediated signaling.
• Viable T cells in normal or TCR transgenic mice did not show parallel rAnV binding.
• BM stromal cells influence rAnV binding by viable BM B cells in vitro.

Conclusions:

• rAnV detects a membrane alteration in developing and mature B cells associated with BCR selection.
• This membrane change is specific to B cells and not observed in viable T cells.
• Stromal cell interactions play a role in modulating this B cell membrane alteration.