Schuh and colleagues (1) recently reported that a transgenic Sp6-μHC can be transported to the cell surface in the absence of surrogate and conventional light chains (rag-deficient background). They ascribe this property to the Sp6-variable region, and speculate that the production of some μ chains with particular variable regions could explain the “leaky” phenotype of λ5-deficient mice. We have found that a transgenic Sp6-human μ heavy chain was inefficient in correcting pre-B cell development in rag-proficient, λ5-deficient mice, while a V-less human μ chain relieved the requirement for surrogate light chains (2). Our data suggest that there is nothing particular with the Sp6 variable region and that the good efficiency in promoting pre-B cell development in the rag-deficient background might be related to homeostatic compensation due to Ig and B cell deficiency. Another interpretation of their data, that we favor, is that any rearranged μ gene can give rise to a minority of VDJ-less transcripts by splicing of the leader exon to the CH1 exon, which, in mouse as in man, can induce V-less protein production (3, 4). This is consistent with the low expression of μ chain on the surface. In the transgenic model described by Schuh and colleagues, the presence or the absence of variable region on the B cell surface should be tested.
The Authors Respond: Signaling and Cell Surface Expression of μH Chains in the Absence of Surrogate and Conventional Light Chains
In his comments about our manuscript entitled “Cutting Edge: Signaling and Cell Surface Expression of a μH Chain (μHC) in the Absence of λ5: A Paradigm Revisited” (R1 ), Corcos argues that the positive effect of the transgenic Sp6-μHC on the progression of cells from the pro-B to the pre-B cell stage in the absence of λ5 might rather be the result of signals delivered by a truncated μHC lacking the VH region (V-less μHC) than by the wild-type Sp6-μHC.
The idea that truncated VH-less μHCs promote the transition of cells from the pro-B to the pre-B stage even in the absence of a complete surrogate light chain (SLC) has been verified in transgenic mouse models by Corcos and coworkers (R2 ) as well as Schlissel and coworkers (R3 ). However, in our system we can exclude the presence of truncated VH-less μHCs, since, as already published in an earlier manuscript by Hess et al. (see Fig. 2D in Ref. R4 ), we detected in bone marrow B-lymphoid cells by Western blot analysis only a signal corresponding to a 70-kDa full-length μHC but never a signal indicating the presence of a shorter chain. Therefore, developmental progression of pro-B cells in the absence of λ5 is driven in our transgenic mouse model by a full-length wild-type Sp6-μHC rather than a VH-less μHC.
In his comments, Corcos also mentioned that a chimeric μHC composed of the mouse VHSp6 region and the human Cμ region (chimeric mouse/human Sp6-μHC) was inefficient in promoting pre-B cell development in λ5-deficient, Rag-proficient mice (R2 ). This statement is somewhat misleading, since the contour blots in Fig. 4 in Ref. R2 clearly show that the chimeric mouse/human Sp6-μHC supports the progression of pro-B cells into CD43-negative pre-B cells in transgenic λ5-deficient mice when compared with nontransgenic λ5-deficient mice, albeit with a lower efficiency than in λ5-proficient transgenic mice. Therefore, the findings by Corcos et al. (R2 ) are in line with our findings and support the conclusion that a full-length Sp6-μHC promotes progression of pro-B cells in the absence of a λ5 chain, although with a lower frequency than in the presence of λ5.
We also think that the positive effect of a transgenic Sp6-μHC on pro-B cell progression in the absence of λ5 is due to a structural feature of the VHSp6 region, since Nussenzweig and coworkers found that a chimeric mouse/human μHC using another murine VH region (i.e., VH3–38; Ref. R5 ) does not support the progression of pro-B cells in a transgenic 3-38-μHC, λ5/Rag-double-deficient mouse (R6 ).
In summary, our findings (R1 ) as well as that of Corcos et al. (R2 ) and Nussenzweig et al. (R6 ), strongly support the idea that the leaky phenotype of B cell maturation in λ5-deficient mice could be explained by the production of some μHCs that gain in a VH-dependent manner surface transport-and signaling competency even in the absence of a complete SLC.