ANALYSIS OF B CELL DEVELOPMENT IN TDT KNOCKOUT MICE

Aging

Investigators

Abstract

We have analyzed the development of B cells in Terminal Deoxynucleotide Transferase (TdT)knockout mice. Two aspects have been studied: 1) the effect on the immune response to phosphocholine (PC) in mice immunized with Proteus morganii; and 2) the effect on B1 B cell development. The immune response of normal mice to P. morganii is dominated by B cells expressing a M603 idiotype. Thus, the V1 H chain of these antibodies has a somatic mutation at the V:D junction in which the germline encoded Asp at position 96 has been changed to an Asn. It has been claimed that anti-PC antibodies of other idiotypes are not capable of binding to the PC-epitope on P. morganii. However, in TdT knockout mice immunized with this bacterial antigen only germline encoded T15-idiotype anti-PC antibodies are produced. This data demonstrates that TdT is responsible for the Asp to Asn change that occurs in the V1 gene of normal mice and it also shows that the response to P. morganii is not restricted to the M603-idiotype. Published work on TdT knockout mice shows that loss of TdT has very little effect on the animals ability to respond to a wide variety of antigens. This was surprising in that TdT isresponsible for generating diversity in the 3rd hypervariable region of the antibody molecule and this region of the antibody often defines the specificity of the antibody molecule. We found that TdT knockout mice have a higher level of B1 B cell development than normal mice. This increase in CD5+ B cells is accompanied by a 3 fold increase in the number of peritoneal B cells that bind to phosphatidyl choline (PtC). In normal mice up to 50% of the PtC binding cells display the VH-12 idiotype. However, in TdT knockout mice the VH-12 PtC-specific B cellsrepresent a much lower fraction of the PtC pool. These data demonstrate that TdT plays a role in controlling the homeostasis of the B1 B cell pool, and it also affects the H-chain makeup andantigenic specificity of this pool of B cells.