Initiation of the phosphocholine-specific response to Proteus morganii. B cell transfectants expressing unmutated VH/VL can respond to stimulation by P. morganii antigen
Penner, S.J.; George, J.; Claflin, L.
Journal of Immunology 155(5): 2387-2395
1995
ISSN/ISBN: 0022-1767 PMID: 7544373 Document Number: 445796
We wished to resolve a paradox of how the response to the phosphocholine (PC) determinant of Proteus morganii could be initiated from a precursor B cell whose receptor, in unmutated form as Ab, appears to be unable to bind Ag. Unmutated V-H and unmutated V-L constructs were co-transfected into the B cell lymphoma M12.4 to study stimulation via membrane Ig (mIg). The same V-H Construct was expressed in an L+, H- hybridoma line to characterize Ab binding. The unmutated Ab showed no detectable binding in ELISA to the PC-containing Ag from P. morgani PC(PM). By contrast, the unmutated mIg mediated mobilization of calcium in response to the PC(PM) Ag. Single-positive B cell lines or mIgM, mIgD double-positive lines were all capable of responding. The degree of signaling depended greatly on high receptor number, and only a fraction of cells in the population responded. Inhibition of the PC(PM)-induced calcium response by free PC indicated that the response was Ag-specific. A transfectant B cell line expressing moderate levels of a high affinity, mutated mIgM readily responded to PC(PM). These observations indicate that the unmutated Ig as a receptor is capable of interacting with PC(PM) and suggest that the immune response to PC(PM) could originate from the precursor B cell expressing the unmutated mIg. The role of mIgD vs mIgM is discussed in terms of the requirement for high receptor number in the signaling process.