Human B cells secrete predominantly lambda L chains in the absence of H chain expression.

Abstract

Ig H and L chains are independently assembled in B cells and then secreted together as a functional protein. H chains cannot be secreted without assembly to L chains; however, L chains can be secreted in the absence of H chains by both mice and human cells. To examine the influence of H chain expression on human L chain isotype selection (kappa or lambda), we compared the kappa/lambda ratio of L chains unassociated with H chains (free L chains) to the kappa/lambda ratio of L chains associated with H chains. Culture supernatants of human splenocytes were assayed for kappa and lambda L chains. Free L chains were the predominant form of L chains detected in unstimulated cultures, accounting for 68 to 70% of the total. This was in contrast to the minor proportion that free L chains represented (less than 20%) in cultures stimulated with PWM or LPS (p less than 0.01). Furthermore, the kappa/lambda ratio of light chains detected in unstimulated cultures was 0.5 as compared to 1.3 for PWM stimulated cultures (p = 0.0001). To demonstrate that the decreased kappa/lambda ratio of L chains in the supernatants of cultures of unstimulated B cells was due to free L chains, we measured the kappa/lambda ratio of IgG and IgM-associated L chains. In both the stimulated and unstimulated cultures, the kappa/lambda ratio of L chains associated with H chains was greater than the ratio determined for free L chains. Free L chains were shown to be predominantly lambda as compared to the predominantly kappa phenotype of L chains associated with H chains. Thus absence of H chain expression affects selection of L chain isotypes secreted by human B cells.

The dispensability of VH-VL pairing and the indispensability of VL domain integrity in the IgG1 secretion process

“…We also observed that wt LC was secreted in both monomeric and dimeric forms, while C κ was secreted as a dimer ( Figure 4C , lanes 1). It is intriguing that wt LC is secreted as a mixture of these two forms, contrary to previous reports that some LCs are secreted as dimers ( Leitzgen et al, 1997 ) while others are monomers ( Ambrosino et al, 1991 ; Dul et al, 1996 ; Skowronek et al, 1998 ). Secretion of single C κ domains was observed at a level barely detectable in the absence of HC expression, unlike wt LC (V κ -C κ ).…”

“…Conversely, most LCs can be secreted in free form without the need for HC assembly ( Ambrosino et al, 1991 ; Dul et al, 1996 ; Leitzgen et al, 1997 ; Skowronek et al, 1998 ). While certain LCs require dimerization for proper folding and secretion ( Leitzgen et al, 1997 ), many LCs can be secreted as monomers ( Ambrosino et al, 1991 ; Dul et al, 1996 ; Skowronek et al, 1998 ). Most free LCs bind transiently to BiP through their V L domain, in contrast to HCs where BiP associates stably with unfolded and reduced C γ1 .…”