Molecular basis of the cell-surface expression of immunoglobulin mu chain without light chain in human B lymphocytes.

Abstract

Four distinct human B-lymphoid cell lines possess the ability to circumvent the mechanism regulating intracellular transport of immunoglobulin protein. These cells do not produce light chains, yet they express ,u heavy chains on the cell surface at comparable levels to B-cell lines that produce native forms of both proteins. The IL-chain mRNA produced in all four cell lines was found to contain an identical deletion of most of the heavy-chain variable (VH) region (75% of the 3' portion), with no apparent alteration in constant (C) region structure. The truncated gL (Iu*)-chain mRNA in these cells was created through the use of a cryptic splice donor site found within the human VH gene(s) utilized by these B-cell lines. The truncated IL* chains exhibited a decreased ability to associate with the intracellular transport regulatory protein, heavychain binding protein (BiP). This result indicates that VH region structure, in addition to C,,1 region structure, influences the formation of the BiP recognition site on the heavy chain. Furthermore, it suggests that the mechanism allowing for cell-surface expression of the p* chains in the absence of light-chain pairing is the inability of BiP to bind to the IL* chains and hence prevent their intracellular transport. The high frequency with which the ,I-only surface immunoglobulin positive phenotype is present in our collection of human B-cell lines and the isolation of one of the cell lines from a healthy individual also suggest that B cells of this type may represent a significant subpopulation among the normal human B-cell repertoire.In the developmental pathway for mammalian B lymphocytes, the immunoglobulin heavy (H)-chain-only phenotype is normally limited to the precursor population known as pre-B cells, which generally synthesize the ,t form of H chain (1-3). Certain instances can arise though where more mature B-lymphoid cells can also stably express H chains in the absence of any light (L)-chain synthesis. The best documented examples of this phenomenon are found in plasma cell stage lines isolated from patients with 'y H-chain disease (4) or represented by certain variant clones isolated during culture of mouse plasmacytomas (5). The H chain produced by the plasmacytoid cell lines from each of these sources is always found to be structurally altered from the native H-chain protein of the same isotype and is generally secreted by the cell as an H-chain-only multimeric complex. The ability of these altered H chain (H*) to be transported intracellularly in the absence of L-chain association contrasts with the block in intracellular transport for native ,4 chains in pre-B cells (2).An explanation to this anomaly has been recently provided by studies showing that H-chain transport in B-lymphoid cells is regulated by an endoplasmic reticulum (ER)-localized protein termed H-chain binding protein (BiP; see ref. 6), which apparently belongs to the glucose-regulated family of proteins (7). BiP appears to regulate transport of several membrane and secreted glycopr...

Analysis in vivo of GRP78-BiP/substrate interactions and their role in induction of the GRP78-BiP gene.

“…However, the nature of the deletions makes it possible that within these large segments of HN more than one site is available to interact with GRP78-BiP, but these remain to be determined. Naturally occurring deletions of these domains reduce the association with GRP78-BiP and allow the secretion of HC in the absence of light-chain synthesis Pollock et al, 1987). However, these observations should not be necessarily interpreted to suggest that GRP78-BiP is absent from the process of folding of mutant HC because either deletion mutant may continue to contain the other available binding sites.…”

Regulating the retention of T-cell receptor alpha chain variants within the endoplasmic reticulum: Ca(2+)-dependent association with BiP.

“…Previous workby Hendershot et al (1987) and Pollok et al (1987) has provided strong evidence that the stable retention of newly synthesized Ig heavy chains within the ER is a consequence oftheir association with BiR They have shown that large deletions in either the first constant region domain (CHI) or in the variable region (VH) of Ig heavy chains result in proteins that do not interact with BiP The lack of BiP binding is correlated with secretion of CH, mutants and the surface expression of VH mutants.…”

Immunoglobulin gamma 2b transgenes inhibit heavy chain gene rearrangement, but cannot promote B cell development.

“…The association between X5 and j. heavy chains seems to occur through the CH1 domain of it, as deletions of this domain result in loss of association between /,4 and these polypeptides (43). Based on the conservation ofBiP binding and light chain binding to y2b via associations within the CH1 domain, it seems reasonable to expect that X5, which is structurally similar to a light chain, may also associate with y2b (44)(45)(46)(47) . However, even if y2b can bind X5, a correct signal for B cell maturation may not be created.…”