B Lymphocyte Development in Rabbit: Progenitor B Cells and Waning of B Lymphopoiesis

In mammals that use gut-associated lymphoid tissues for expansion and somatic diversification of the B cell repertoire, B lymphopoiesis occurs early in ontogeny and does not appear to continue throughout life. In these species, including sheep, rabbit, and cattle, little is known about the pathway of B cell development and the time at which B lymphopoiesis wanes. We examined rabbit bone marrow by immunofluorescence with anti-CD79a and anti-mu and identified both proB and preB cells. The proB cells represent the vast majority of B-lineage cells in the bone marrow at birth and by incorporation of 5-bromo-2'-deoxyuridine, they appear to be a dynamic population. PreB cells reach maximum levels in the bone marrow at 3 wk of age, and B cells begin to accumulate at 7 wk of age. We cloned two VpreB and one lambda5 gene and demonstrated that they are expressed within B-lineage cells in bone marrow. VpreB and lambda5 coimmunoprecipitated with the mu-chain in lysates of 293T cells transfected with VpreB, lambda5, and mu, indicating that VpreB, lambda5, and mu-chains associate in a preB cell receptor-like complex. By 16 wk of age, essentially no proB or preB cells are found in bone marrow and by PCR amplification, B cell recombination excision circles were reduced 200-fold. By 18 mo of age, B cell recombination excision circles were reduced 500- to 1000-fold. We suggest that B cell development in the rabbit occurs primarily through the classical, or ordered, pathway and show that B lymphopoiesis is reduced over 99% by 16 wk of age.

A single 3' alpha hs1,2 enhancer in the rabbit IgH locus

Multiple cis-acting elements including the intronic enhancer and the 3'alpha enhancer (3'alphaE) regulate expression of the Ig heavy chain genes during B cell development. A 3'alphaE is composed of DNase I-hypersensitive sites, hs1,2, hs3a,b, and hs4, found 3' of the murine Calpha gene as well as 3' of both human Calpha genes, Calpha1 and Calpha2. Rabbits have 13 Calpha genes, and we tested whether a 3'alphaE is associated with each of these genes. To identify 3'alphaE regions we developed a rabbit hs1,2 probe and used this to search for enhancer homologues of human hs1,2 in a genomic fosmid library. We identified a single hs1,2 fragment 8-kb downstream of Calpha13, the presumed 3'-most Calpha gene. We also identified and partially sequenced a new Calpha gene, Calpha14, located 6 kb upstream of Calpha13. Genomic Southern blot analysis confirmed that the rabbit genome contains only one hs1,2 enhancer region. We tested the enhancer activity of the hs1,2 with the SV40, V(H), and Ialpha promoters using the luciferase reporter gene in transient transfection assays and found that it significantly enhanced the activity of SV40 and V(H) promoters and slightly enhanced an Ialpha promoter. We conclude that the rabbit has a single hs1,2 enhancer that resides at the 3' end of the IgH gene cluster and may constitute one of the cis-elements regulating the expression of IgH genes.

Trans-chromosomal recombination within the Ig heavy chain switch region in B lymphocytes

Somatic DNA rearrangements in B lymphocytes, including V(D)J gene rearrangements and isotype switching, generally occur in cis, i. e., intrachromosomally. We showed previously, however, that 3 to 7% of IgA heavy chains have the VH and Calpha regions encoded in trans. To determine whether the trans-association of VH and Calpha occurred by trans-chromosomal recombination, by trans-splicing, or by trans-chromosomal gene conversion, we generated and analyzed eight IgA-secreting rabbit hybridomas with trans-associated VH and Calpha heavy chains. By ELISA and by nucleotide sequence analysis we found that the VH and Calpha regions were encoded by genes that were in trans in the germline. We cloned the rearranged VDJ-Calpha gene from a fosmid library of one hybridoma and found that the expressed VH and Calpha genes were juxtaposed. Moreover, the juxtaposed VH and Calpha genes originated from different IgH alleles. From the same hybridoma, we also identified a fosmid clone with the other expected product of a trans-chromosomal recombination. The recombination breakpoint occurred within the Smicro/Salpha region, indicating that the trans-association of VH and Calpha genes occurred by trans-chromosomal recombination during isotype switching. We conclude that trans-chromosomal recombination occurs at an unexpectedly high frequency (7%) within the IgH locus of B lymphocytes in normal animals, which may explain the high incidence of B-cell tumors that arise from oncogene translocation into the IgH locus.

Evidence for limited B-lymphopoiesis in adult rabbits

Rabbits are born with a limited VDJ gene repertoire formed primarily by rearrangement of one VH gene, VH1. The VDJ genes are undiversified at birth but become diversified by approximately 2 mo of age. To investigate more closely the time during which this diversity occurs, we determined the nucleotide sequences of VDJ genes from peripheral blood leukocytes taken from young rabbits at various time points, and we examined the extent of the diversification of the VDJ genes. At 4 wk of age there were, on average, 3 nucleotide changes per VH region, with approximately 75% of the genes showing some diversification. The number of nucleotide changes per VH region increased to 12 by 6-8 wk of age, and all but 1 of the 35 sequences analyzed were diversified. Because only a limited number of genes can be examined by nucleotide sequence analysis, we used an RNase protection assay to examine a large number of genes and we determined the level of undiversified VH1 mRNA in lymphoid organs of both young and adult rabbits. In young rabbits, we found a high level of undiversified VDJ genes, but the level was greatly reduced by 2 mo of age. By adulthood, essentially all VDJ genes of cells from appendix, peripheral blood, and bone marrow were diversified. Because we had expected B lymphopoiesis to be ongoing in the bone marrow of adult rabbits, we were surprised not to find undiversified VDJ genes from the newly generated B cells. Therefore, we searched for evidence of ongoing B lymphopoiesis in bone marrow by isolating and examining circular DNA for the presence of VD and DJ recombination signal joints. We found highly reduced levels of recombination signal joints in bone marrow of adult rabbits relative to the levels found in bone marrow of newborn rabbits. These data indicate that limited VD and DJ gene rearrangements occur in bone marrow of adult rabbits, and we therefore suggest that B lymphopoiesis is limited in adults.

Transchromosomally derived Ig heavy chains

During an immune response, activated B cells undergo isotype switching and begin to express isotypes other than IgM and IgD. Isotype switching occurs when downstream C gamma, C alpha, or C epsilon genes are rearranged into the S mu chromosomal region, resulting in the deletion of the region in between. These rearrangements usually occur in cis, i.e., intrachromosomally. In previous studies, we analyzed allotypic specificities of rabbit secretory IgA and identified a substantial number of IgA heavy chains with VH and C alpha allotypes that were encoded by VH and C alpha genes in trans. In those studies, however, we could not determine whether the trans association of VH and C alpha occurred during VDJ gene rearrangement or during isotype switching. Here, we cloned rabbit cDNA which encodes these trans IgA heavy chains and determined the chromosomal origin of the VH, JH, and C alpha regions. To determine whether the trans association occurred during VDJ gene rearrangement, we analyzed the nucleotide polymorphism of the JH region and the VH allotype encoded by the cDNA. We found that the VH and JH genes used in the VDJ gene rearrangements were from the same chromosome, indicating that the VH, D, and JH gene rearrangements occurred in cis. Furthermore, we analyzed the DNA polymorphisms of JH and C alpha and showed that the VDJ and C alpha genes encoding the trans IgA molecules were derived from different parental chromosomes. We suggest that the trans association occurred during isotype switching. This study shows that VH and CH can associate transchromosomally as part of a normal immune response.

Transchromosomally derived Ig heavy chains

During an immune response, activated B cells undergo isotype switching and begin to express isotypes other than IgM and IgD. Isotype switching occurs when downstream C gamma, C alpha, or C epsilon genes are rearranged into the S mu chromosomal region, resulting in the deletion of the region in between. These rearrangements usually occur in cis, i.e., intrachromosomally. In previous studies, we analyzed allotypic specificities of rabbit secretory IgA and identified a substantial number of IgA heavy chains with VH and C alpha allotypes that were encoded by VH and C alpha genes in trans. In those studies, however, we could not determine whether the trans association of VH and C alpha occurred during VDJ gene rearrangement or during isotype switching. Here, we cloned rabbit cDNA which encodes these trans IgA heavy chains and determined the chromosomal origin of the VH, JH, and C alpha regions. To determine whether the trans association occurred during VDJ gene rearrangement, we analyzed the nucleotide polymorphism of the JH region and the VH allotype encoded by the cDNA. We found that the VH and JH genes used in the VDJ gene rearrangements were from the same chromosome, indicating that the VH, D, and JH gene rearrangements occurred in cis. Furthermore, we analyzed the DNA polymorphisms of JH and C alpha and showed that the VDJ and C alpha genes encoding the trans IgA molecules were derived from different parental chromosomes. We suggest that the trans association occurred during isotype switching. This study shows that VH and CH can associate transchromosomally as part of a normal immune response.

Limited repertoire of utilized VH gene segments in a VHa3-allotype-suppressed rabbit

Between 70 and 90% of serum Ig molecules of normal laboratory rabbits bear one of the serologically defined VHa allotypic specificities, a1, a2, or a3, and are termed VHa+ (a-positive) molecules; the remaining 10-30% of Ig molecules that do not have VHa allotypic specificities are designated VHa- (a-negative). The repertoire of utilized VHa(+)-encoding gene segments has been examined extensively, but only limited studies of the repertoire of utilized VHa(-)-encoding gene segments in normal rabbits have been done. To examine the repertoire of utilized VHa- gene segments, we analyzed VH-encoding cDNA clones from mRNA of a VHa-allotype-suppressed rabbit whose serum Ig was primarily VHa-. For VHa suppression a newborn a3/a3 rabbit was injected periodically with anti-VHa3 antiserum; when it was 2 months of age and its serum Ig was greater than 94% VHa-, cDNA clones were generated from splenic RNA. The nucleotide sequences of eight putative VHa(-)-encoding cDNA clones were compared to those of eight cDNA clones generated from RNA of non-suppressed a3/a3 rabbits. The presumed VHa3-encoding cDNA clones from the non-suppressed rabbits appeared to derive from VH1-a3, the 3'-most germline VH gene segment. In contrast, the VHa(-)-encoding cDNA clones from the suppressed rabbit were distinctly different from VH1 and were most probably derived from germline VH segments other than VH1. Because the expressed VHa- gene repertoire was highly restricted, we propose that VHa- molecules in a3/a3 rabbits may be derived from as few as three germline VH gene segments.

Genetic mapping of the human polymeric immunoglobulin receptor gene to chromosome region 1q31----q41

Polymeric immunoglobulin receptor (PIGR) is a transmembrane glycoprotein which is expressed by epithelial cells and is involved in the transcellular transport of polymeric immunoglobulins into secretions. We cloned the human gene for PIGR and used the clone to obtain probes to determine the chromosomal localization of PIGR. Analysis of somatic cell hybrids and in situ chromosomal hybridization localized the human PIGR gene locus to 1q31----q41.

Rabbit secretory components: identification of a third allotype, t63

A third allotype of rabbit secretory component has been identified. The allotype previously referred to as t62 by our laboratory can now be subdivided into two allotypes, t62 and t63, with alloantisera capable of discriminating between the two. Results of family studies are consistent with a three allele system (t61, t62 and t63) at the t-locus. By SDS PAGE, electrophoretic mobilities of the multiple SC bands for each of the three allotypes are characteristic of the allotype; the apparent molecular sizes of the bands of the t62 allotype are 2 to 3 kDa lower than those for the t61 allotype. The banding patterns of the t61 and t63, although similar, are not identical to each other. Results of serologic cross-reaction studies and of tryptic peptide mapping studies suggest multiple structural differences between the allotypes as well as a closer relationship between t62 and t63 than between either of these allotypes and t61.

Rabbit secretory components: identification of a third allotype, t63

A third allotype of rabbit secretory component has been identified. The allotype previously referred to as t62 by our laboratory can now be subdivided into two allotypes, t62 and t63, with alloantisera capable of discriminating between the two. Results of family studies are consistent with a three allele system (t61, t62 and t63) at the t-locus. By SDS PAGE, electrophoretic mobilities of the multiple SC bands for each of the three allotypes are characteristic of the allotype; the apparent molecular sizes of the bands of the t62 allotype are 2 to 3 kDa lower than those for the t61 allotype. The banding patterns of the t61 and t63, although similar, are not identical to each other. Results of serologic cross-reaction studies and of tryptic peptide mapping studies suggest multiple structural differences between the allotypes as well as a closer relationship between t62 and t63 than between either of these allotypes and t61.

Structural variability of rabbit secretory components. Allotype-associated differences in the third, fourth, and fifth domains

We have previously shown (Frutiger, S., Hughes, G. J., Hanly, W. C., Kingzette, M., and Jaton, J.-C. (1986) J. Biol. Chem. 261, 16673-16681) that limited tryptic digestion of the high Mr form of rabbit secretory component of allotypes t61, t62, and t63 generates two major fragments, the NH2-terminal domain and a 40-kDa fragment encompassing domains 3, 4, and 5. Similarly, from the low Mr form of secretory component, (SC) the NH2-terminal domain, together with a 30-kDa fragment containing domains 4 and 5, were released. These fragments were used as inhibitors in a sensitive competitive binding radioimmunoassay with noncross-reactive rabbit alloantisera to study the distribution and localization of the major allotype-specific allotopes within the SC polypeptide. The 40-kDa fragments were shown to inhibit the 125I-labeled intact SC/anti-SC allotype reaction to the extent of 90%, i.e. nearly as well as the intact homologous high Mr SC form. In contrast, the NH2-terminal fragments (domain 1) were not inhibitory. The low Mr SC of each allotype was less inhibitory on a molar basis than the homologous high Mr SC polypeptide, an observation compatible with the deletion of domains 2 and 3 in the smaller polypeptide (Deitcher, D. L., and Mostov, K. E. (1986) Mol. Cell. Biol. 6, 2712-2715; Frutiger, S., Hughes, G. J., Fonck, Ch., and Jaton, J.-C. (1987) J. Biol. Chem. 262, 1712-1715). The structural correlates of the allotypic specificities were evaluated by comparative peptide mapping of the 40-kDa fragments (allotypes t61, t62, and t63). The data suggest that the t61 allotype structure differs significantly from the t62 and t63 structures, the latter two being much more related to each other than to t61. These findings are in full agreement with the serological data. The inhibition results suggest that the major allotype-specific, noncross-reactive allotopes of SC are distributed throughout domains 3, 4, and 5, even though domain 4 appears to be more conserved than domains 3 and 5 between the allotypes t61 and t63. Seven amino acid substitutions between t61 and t63 have been detected within domains 3, 4, and 5.

The amino-terminal domain of rabbit secretory component is responsible for noncovalent binding to immunoglobulin A dimers

Rabbit secretory components (SC) constitute a family of markedly heterogeneous glycoproteins which are released in the secretions as free SC or as SC bound to polymeric immunoglobulins. The aim of this work was to determine the region of the SC polypeptides which is involved in IgA binding. The high and the low Mr forms of free SC (or IgA-dissociated bound SC) and the native secretory IgA complex were subjected to limited tryptic digestion. Chemically characterized peptides ranging in apparent size from 15 to 20 kDa, depending upon the allotype, were shown to be necessary and sufficient for efficient noncovalent binding to IgA dimers (subclass g). These fragments encompass the amino-terminal first domain of SC, i.e. residues 1-126, when aligned with the predicted amino acid sequence from a cDNA clone encoding the rabbit polymeric Ig receptor (Mostov, K.E., Friedlander, M., and Blobel, G. (1984) Nature 308, 37-43). The high and the low Mr forms of SC exhibited the same relative affinity for IgA dimers, suggesting that the postulated internal deletion in the smaller polypeptide (Kühn, L. C., Kocher, H.-P., Hanly, W.C., Cook, L., Jaton, J.-C., and Kraehenbuhl, J.-P. (1983) J. Biol. Chem. 258, 6653-6659) does not impair the IgA dimer recognition function.

Increased activation of the alternative complement pathway in sickle cell disease

Complement proteins play an important role in host defenses against Streptococcus pneumoniae, a major cause of serious infections in sickle cell (SS) disease. Previous studies have suggested abnormalities of the alternative complement pathway in SS disease. We measured activation of the alternative pathway in sera from patients with SS disease utilizing an enzyme immunoassay which detects C3b,P complexes, derivative of the C3b,Bb,P alternative pathway convertase. In all, 89% of SS sera had elevated concentrations of C3b,P complexes, indicative of increased alternative pathway activation. Chronic activation of the alternative pathway may contribute to impaired host defense in SS patients.

C-reactive protein inhibits pneumococcal activation of the alternative pathway by increasing the interaction between factor H and C3b

We previously studied two alternative pathway activators, Streptococcus pneumoniae and positively charged liposomes, which react with C-reactive protein (CRP). Binding of CRP to these surfaces initiates classical pathway but blocks alternative pathway activation. In this study, we investigated the mechanism of this inhibition using S. pneumoniae, R36a. R36a were pretreated with CRP (CRP-R36a) or buffer and incubated with C2-deficient human serum to which 125I-labeled C3 had been added. The amount of specific 125I-C3 binding was decreased from 8200 mol/CFU on R36a to 2200 mol/CFU on CRP-R36a. In contrast, when the same experiment was performed with purified factors B, D, P, and C3, in the absence of regulatory proteins, specific 125I-C3 uptake was slightly lower on R36a (6100 mol/CFU) than on CRP-R36a (8100 mol/CFU). The ability of the fixed C3b to inactivate factor B in the presence of factor D was equivalent on the two surfaces. The binding of the regulatory factor H to C3b fixed to R36a and CRP-R36a was compared by using purified 125I-labeled factor H. The ratio of factor H bound to C3 bound was twofold greater on CRP-R36a than on R36a. This increase was found by using C2-deficient serum or purified factors B, D, P, and C3 to fix C3b to the surfaces. The ability of CRP to inhibit C3 binding to R36a was restored by the addition of factors H and I to factors B, D, P, and C3. These results indicate that CRP inhibits alternative pathway activation by increasing regulation of bound C3.

C-reactive protein inhibits pneumococcal activation of the alternative pathway by increasing the interaction between factor H and C3b

We previously studied two alternative pathway activators, Streptococcus pneumoniae and positively charged liposomes, which react with C-reactive protein (CRP). Binding of CRP to these surfaces initiates classical pathway but blocks alternative pathway activation. In this study, we investigated the mechanism of this inhibition using S. pneumoniae, R36a. R36a were pretreated with CRP (CRP-R36a) or buffer and incubated with C2-deficient human serum to which 125I-labeled C3 had been added. The amount of specific 125I-C3 binding was decreased from 8200 mol/CFU on R36a to 2200 mol/CFU on CRP-R36a. In contrast, when the same experiment was performed with purified factors B, D, P, and C3, in the absence of regulatory proteins, specific 125I-C3 uptake was slightly lower on R36a (6100 mol/CFU) than on CRP-R36a (8100 mol/CFU). The ability of the fixed C3b to inactivate factor B in the presence of factor D was equivalent on the two surfaces. The binding of the regulatory factor H to C3b fixed to R36a and CRP-R36a was compared by using purified 125I-labeled factor H. The ratio of factor H bound to C3 bound was twofold greater on CRP-R36a than on R36a. This increase was found by using C2-deficient serum or purified factors B, D, P, and C3 to fix C3b to the surfaces. The ability of CRP to inhibit C3 binding to R36a was restored by the addition of factors H and I to factors B, D, P, and C3. These results indicate that CRP inhibits alternative pathway activation by increasing regulation of bound C3.

Activation of Human Complement by Liposomes: A Model for Membrane Activation of the Alternative Pathway

Liposomal model membranes were found to activate the alternative pathway of human complement. Activation was measured by C3 conversion and component consumption in serum that had been incubated with liposomes. C3 conversion did not require C1 or C2 of the classical pathway, since it was observed in serum from a C1r-deficient patient, serum from a C2-deficient patient, and normal serum in buffer containing EGTA and MgCl2. The incubation of liposomes with C2-deficient serum resulted in consumption of components C3 through C9 with no consumption of C1 or C4 in a profile typical of alternative pathway activation. The reaction was further shown to require alternative pathway factor D, and to be independent of antibody.

Activation of the alternative pathway was dependent on the membrane composition of the liposomes. A positive charge was required for liposomes to produce C3 conversion. Liposomal cholesterol concentration and phospholipid fatty acyl chain length and unsaturation all influenced activation, suggesting the importance of membrane fluidity. Positively charged liposomes containing dimyristoyl phosphatidylcholine and cholesterol required the presence of certain glycolipids for C3 conversion. The activation of the alternative complement pathway by liposomes of defined membrane composition may provide a suitable model for the study of alternative pathway activation by cellular membranes.

Activation of human complement by liposomes: a model for membrane activation of the alternative pathway

Liposomal model membranes were found to activate the alternative pathway of human complement. Activation was measured by C3 conversion and component consumption in serum that had been incubated with liposomes. C3 conversion did not require C1 or C2 of the classical pathway, since it was observed in serum from a C1r-deficient patient, serum from a C2-dificient patient, and normal serum in buffer containing EGTA and MgCl2. The incubation of liposomes with C2-deficient serum resulted in consumption of components C3 through C9 with no consumption of C1 or C4 in a profile typical of alternative pathwya activation. The reaction was further shown to require alternative pathway factor D, and to be independent of antibody. Activation of the alterative pathway was dependent on the membrane composition of the liposomes. A positive charge was required for liposomes to produce C3 conversion. Liposomal cholesterol concentration and phospholipid fatty acyl chain length and unsaturation all influenced activation, suggesting the importance of membrane fluidity. Positively charged liposomes containing dimyristoyl phosphatidylcholine and cholesterol required the presence of certain glycolipids for C3 conversion. The activation of the alternative complement pathway by liposomes of defined membrane composition may provide a suitable model for the study of alternative pathway activation by cellular membranes.