lambda 5, but not mu, is required for B cell maturation in a unique gamma 2b transgenic mouse line

Pulmonary vascular inflammation with fatal coronavirus disease 2019 (COVID-19): possible role for the NLRP3 inflammasome

Background

Pulmonary hyperinflammation is a key event with SARS-CoV-2 infection. Acute respiratory distress syndrome (ARDS) that often accompanies COVID-19 appears to have worse outcomes than ARDS from other causes. To date, numerous lung histological studies in cases of COVID-19 have shown extensive inflammation and injury, but the extent to which these are a COVID-19 specific, or are an ARDS and/or mechanical ventilation (MV) related phenomenon is not clear. Furthermore, while lung hyperinflammation with ARDS (COVID-19 or from other causes) has been well studied, there is scarce documentation of vascular inflammation in COVID-19 lungs.

Methods

Lung sections from 8 COVID-19 affected and 11 non-COVID-19 subjects, of which 8 were acute respiratory disease syndrome (ARDS) affected (non-COVID-19 ARDS) and 3 were from subjects with non-respiratory diseases (non-COVID-19 non-ARDS) were H&E stained to ascertain histopathological features. Inflammation along the vessel wall was also monitored by expression of NLRP3 and caspase 1.

Results

In lungs from COVID-19 affected subjects, vascular changes in the form of microthrombi in small vessels, arterial thrombosis, and organization were extensive as compared to lungs from non-COVID-19 (i.e., non-COVID-19 ARDS and non-COVID-19 non-ARDS) affected subjects. The expression of NLRP3 pathway components was higher in lungs from COVID-19 ARDS subjects as compared to non-COVID-19 non-ARDS cases. No differences were observed between COVID-19 ARDS and non-COVID-19 ARDS lungs.

Conclusion

Vascular changes as well as NLRP3 inflammasome pathway activation were not different between COVID-19 and non-COVID-19 ARDS suggesting that these responses are not a COVID-19 specific phenomenon and are possibly more related to respiratory distress and associated strategies (such as MV) for treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-022-01944-8.

Independent Roles of Switching and Hypermutation in the Development and Persistence of B Lymphocyte Memory

Somatic hypermutation (SHM) and class-switch recombination (CSR) increase the affinity and diversify the effector functions of antibodies during immune responses. Although SHM and CSR are fundamentally different, their independent roles in regulating B cell fate have been difficult to uncouple because a single enzyme, activation-induced cytidine deaminase (encoded by Aicda), initiates both reactions. Here, we used a combination of Aicda and antibody mutant alleles that separate the effects of CSR and SHM on polyclonal immune responses. We found that class-switching to IgG1 biased the fate choice made by B cells, favoring the plasma cell over memory cell fate without significantly affecting clonal expansion in the germinal center (GC). In contrast, SHM reduced the longevity of memory B cells by creating polyreactive specificities that were selected against over time. Our data define the independent contributions of SHM and CSR to the generation and persistence of memory in the antibody system.

IgG1 B cell receptor signaling is inhibited by CD22 and promotes the development of B cells whose survival is less dependent on Ig alpha/beta

We describe a mouse strain in which B cell development relies either on the expression of membrane-bound immunoglobulin (Ig) gamma1 or mu heavy chains. Progenitor cells expressing gamma1 chains from the beginning generate a peripheral B cell compartment of normal size with all subsets, but a partial block is seen at the pro- to pre-B cell transition. Accordingly, gamma1-driven B cell development is disfavored in competition with developing B cells expressing a wild-type (WT) IgH locus. However, the mutant B cells display a long half-life and accumulate in the mature B cell compartment, and even though partial truncation of the Ig alpha cytoplasmic tail compromises their development, it does not affect their maintenance, as it does in WT cells. IgG1-expressing B cells showed an enhanced Ca(2+) response upon B cell receptor cross-linking, which was not due to a lack of inhibition by CD22. The enhanced Ca(2+) response was also observed in mature B cells that had been switched from IgM to IgG1 expression in vivo. Collectively, these results suggest that the gamma1 chain can exert a unique signaling function that can partially replace that of the Ig alpha/beta heterodimer in B cell maintenance and may contribute to memory B cell physiology.

Scarcity of lambda 1 B cells in mice with a single point mutation in C lambda 1 is due to a low BCR signal caused by misfolded lambda 1 light chain

The presence of valine-154 instead of glycine in the constant region of lambda1 causes a severe lambda1 B cell defect in SJL and lambda1-valine knock-in mice with a compensatory increase in lambda2,3 B cells. The defect is due to low signaling by the lambda1-valine BCR. lambda1-Valine B cells deficient in the SHP-1 phosphatase survive better than lambda2,3 B cells in these mice, or lambda1 B cells in lambda1 wildtype mice. Low signaling is apparently due to misfolding of the lambda1-valine light chain as demonstrated by the absence of a regular beta-sheet structure determined by circular dichroism, the sedimentation of the light chain in solution, and the association of valine-valine constant regions in a yeast two-hybrid assay. lambda1-Valine B cells that survive apparently have a higher BCR signal, presumably because of their specific lambda1-heavy chain combination or having encountered a high-affiniy antigen. lambda1-Valine mice have increased B1 cells which were shown by others to have a higher signaling potential. Valine mice crossed with non-conventional gamma2b transgenic mice, in which B cell development is accelerated and in which B1 cells and high signaling cells are greatly reduced, have essentially no, lambda2,3 B cells, but increased numbers of lambda1-valine B cells. This supports the conclusion that the major defect in lambda1-valine mice is the inability of valine-preB cells to produce a threshold signal for B cell development. The reduction of lambda2,3 B cells in valine mice with a gamma2b transgene shows that the majority of their compensatory increase is almost entirely of the B1 cell type.

Novel B cell population producing functional IgG in the absence of membrane IgM expression

Surface expression of IgM is a characteristic feature of the development of most B cells. Only pre-B cells bearing functional IgM heavy chains mu chains) are selected for clonal expansion and differentiation. Cells lacking mu chains are normally eliminated. muMT mice carrying a deletion of the first exon coding for the transmembrane domain of the immunoglobulin mu chain gene were described as mice deficient for mature B cells, plasma cells and immunoglobulins in serum. In this study, we describe in muMT/BALB/c mice the presence of a novel B cell population, producing IgG, IgA and IgE in the absence of IgM membrane expression. Moreover, this small population of B cells is able to recognize antigens and to differentiate into plasma cells. These "non-conventional" mu(- / -) B cells produce functional immunoglobulins after immunization, undergo germinal center reactions, and maintain B cell memory. Our findings support the concept, that a small percentage of mu -non-expressing pre-B cells can escape elimination, switch to downstream immunoglobulin heavy chains and respond to antigens. It remains an open question how the reactivity of these B cells is regulated and in which extent such B cells play a role in physiological and pathological processes such as autoantibody production and autoimmunity.

A VH12 transgenic mouse exhibits defects in pre-B cell development and is unable to make IgM+ B cells

V(H)12 B cells undergo stringent selection at multiple checkpoints to favor development of B-1 cells that bind phosphatidylcholine. Selection begins with the V(H) third complementarity-determining region (CDR3) at the pre-B cell stage, in which most V(H)12 pre-B cells are selectively eliminated, enriching for those with V(H)CDR3s of 10 aa and a fourth position Gly (designated 10/G4). To understand this selection, we compared B cell differentiation in mice of two V(H)12 transgenic lines, one with the favored 10/G4 V(H)CDR3 and one with a non-10/G4 V(H)CDR3 of 8 aa and no Gly (8/G0). Both H chains drive B cell differentiation to the small pre-BII cell stage, and induce allelic exclusion and L chain gene rearrangement. However, unlike 10/G4 pre-B cells, 8/G0 pre-B cells are deficient in cell division and unable to differentiate to B cells. We suggest that this is due to poor 8/G0 pre-B cell receptor expression and to an inability to form an 8/G0 B cell receptor. Our findings also suggest that V(H)12 H chains have evolved such that association with surrogate and conventional L chains is most efficient with a 10/G4 CDR3. Thus, selection for phosphatidylcholine-binding B-1 cells is most likely the underlying evolutionary basis for the loss of non-10/G4 pre-B cells.

Positive and negative selection of antigen-specific B cells in transgenic mice expressing variant forms of the V(H)1 (T15) heavy chain

Four variant forms of the V1 (T15-H chain) gene are synthesized in mice. Each V1 variant pairs with a distinct L chain to produce a binding site having specificity for phosphocholine (PC). Transgenic mice expressing variant forms of the V1 gene were analyzed to elucidate the factors driving B cell selection into the peripheral repertoire. In all four lines of H chain transgenic mice analyzed, transgene expression caused complete allelic exclusion of endogenous H chains in the bone marrow (BM), whereas most splenic B cells expressed endogenous H chains. The number of sIgM(+) BM B cells and their sIg receptor number was reduced compared to that of normal transgene-negative controls, suggesting that B cells expressing transgene-encoded H chains were being negatively selected in the BM. Mice expressing autoreactive forms of the V1 transgene with lower affinity for PC (M603H and M167H) exhibit positive selection of PC-specific B cells into the spleen, whereas mice expressing the higher affinity T15H variant exhibited elevated PC-specific B cells in the peritoneal cavity but few V(H)1(+) splenic B cells. These data suggest that the higher affinity T15-id(+) B cells preferentially survive in the peritoneal cavity. When these H chain transgenes were crossed into the mu MT knockout mouse in which surface expression of endogenous H chains is blocked, the percent of splenic V(H)1(+) PC-specific B cells increased up to 5-fold and T15-id(+) B cells were detectable in the spleen of T15H mice. This implies that T15-id(+) PC-specific B cells can be selected into the periphery, but they compete poorly with follicular B cells expressing endogenous Ig.

Gene dose-dependent maturation and receptor editing of B cells expressing immunoglobulin (Ig)G1 or IgM/IgG1 tail antigen receptors

Conserved differences between the transmembrane and cytoplasmic domains of membrane immunoglobulin (Ig)M and IgG may alter the function of antigen receptors on naive versus memory B cells. Here, we compare the ability of these domains to signal B cell allelic exclusion and maturation in transgenic mice. A lysozyme-binding antibody was expressed in parallel sets of mice as IgM, IgG1, or a chimeric receptor with IgM extracellular domains and transmembrane/cytoplasmic domains of IgG1. Like IgM, the IgG1 or chimeric IgM/G receptors triggered heavy chain allelic exclusion and supported development of mature CD21(+) B cells. Many of the IgG or IgM/G B cells became CD21(high) and downregulated their IgG and IgM/G receptors spontaneously, resembling memory B cells and B cells with mutations that exaggerate B cell antigen receptor signaling. Unlike IgM-transgenic mice, "edited" B cells that carry non-hen egg lysozyme binding receptors preferentially accumulated in IgG and IgM/G mice. This was most extreme in lines with the highest transgene copy number and diminished in variant offspring with fewer copies. The sensitivity of B cell maturation to transgene copy number conferred by the IgG transmembrane and cytoplasmic domains may explain the diverse phenotypes found in other IgG-transgenic mouse strains and may reflect exaggerated signaling.

The C(H)1 and transmembrane domains of mu in the context of a gamma2b transgene do not suffice to promote B cell maturation

Mice carrying a gamma2b transgene have been shown previously to be deficient in B cell development. In particular, a developmental block exists at the pre-B cell stage. The few B cells that develop all express endogenous micro heavy chains. The phenotype suggests that gamma2b exerts a strong feedback inhibition on endogenous Ig gene rearrangement, but, unlike micro, cannot support further B cell development. In this study we have created hybrid transgenes between gamma2b and micro. Transgenic mice with a C(H)1 domain of micro, or both a C(H)1 and transmembrane/cytoplasmic domain of micro replacing the respective domains of a gamma2b transgene, have the same B cell defect as gamma2b transgenic mice. Interestingly, the severity of the defect is correlated with the level of expression of the transgene, suggesting that the degree of feedback inhibition of Ig gene rearrangement depends on the level and timing of Ig production. Crossing the gamma2b/micro transgenes into a Bcl-x(L) transgenic line allows immature gamma2b B cells to survive, but not to develop to maturity. Therefore, the missing function of micro is not simply an anti-apoptotic effect.

Pre-B-cell development in the absence of lambda 5 in transgenic mice expressing a heavy-chain disease protein

BACKGROUND

Heavy-chain diseases (HCDs) are human lymphoproliferative neoplasias that are characterized by the secretion of truncated immunoglobulin heavy chains devoid of light chains. We have previously proposed--by analogy to the process by which mutated growth factor receptors can be oncogenic--that because the genetic defects in HCDs result in the production of abnormal membrane-associated heavy chains lacking an antigen-binding domain, these abnormal B-cell antigen receptors might engage in ligand-independent signalling. Normal pre-B-cell development requires the presence of the pre-B-cell receptor, formed by the association of mu heavy chains with two polypeptides--so-called surrogate light chains, Vpre-B and lambda 5--that are homologous to the variable and constant portions of immunoglobulin light chains, respectively. To assess whether amino-terminal truncation of membrane-associated heavy chains results in their constitutive activation, we have examined the ability of a HCD-associated mu protein to promote pre-B-cell development in transgenic mice.

RESULTS

When the mu HCD transgene is introduced into SCID mice, CD43- pre-B cells develop normally. To determine whether this pre-B-cell development requires surrogate light chains, we backcrossed mice expressing full-length or truncated mu transgenes with lambda 5-deficient mice. Our results show that the truncated heavy chain, but not the normal chain, is able to promote pre-B-cell development in the absence of lambda 5. We also show that truncated mu chains spontaneously aggregate at the surface of bone marrow cells.

CONCLUSIONS

Expression of the truncated mu heavy chain overrides a tightly controlled step of pre-B-cell development, which strongly suggests that a constitutive signal is delivered by the truncated mu chain disease protein. The self-aggregation of mu chain disease proteins might account for this constitutive activation. We conclude that amino-terminal truncation of heavy chains could play a role in the genesis of HCD neoplasia if it occurs at an appropriate stage of B-cell differentiation, namely in a mature B cell.

Gamma 2b transgenic mice as a model for the role of immunoglobulins in B cell development

The development of B lymphocytes is tightly linked to the expression of immunoglobulins (Igs). Pro/preB cells which do not correctly rearrange heavy/light chain genes are aborted. Correctly rearranged Ig transgenes are apparently recognized by the developing B cells and can prevent the rearrangement of endogenous Ig genes. Both mu and gamma 2b heavy chain genes cause this feedback inhibition of heavy chain gene rearrangement. Mu transgenes can in addition replace endogenous MU in its preB cell survival/maturation function. However, several different transgenic lines have shown that gamma 2b transgenes do not provide the nurturing functions of mu, except for one unique gamma 2b transgenic line, the C line. In this line mature B cells express gamma 2b only. Presumably, an unknown gene has been activated at the transgene integration site whose product overcomes the need for mu. The function of this gene depends of the presence of the surrogate light chain (sL), and thus must operate in combination with the preB cell receptor or in a downstream signaling/antiapoptosis event requiring the gamma 2b/sL receptor. The analysis of the two types of gamma 2b transgenic mice shows that the signals for preB cell development are highly complex and promises to reveal new insights into the molecular and cellular mechanisms of B cell maturation.