Immunoglobulin (Ig) mu, kappa transgenic mice express transgenic idiotype on endogenously rearranged IgM and IgA molecules by secretion of chimeric molecules

Monoclonal gammopathies in aging mu, kappa-transgenic mice: involvement of the B-1 cell lineage

Monoclonal gammopathies (MG) are monoclonal proliferative disorders of B cells at the differentiation stage of Ig production. They can be detected in the serum, either as transient or as persistent homogenous immunoglobulin (H-Ig) components. The exact phenotype, localization, and cell lineage origin of the precursor cells of MG are unknown, but may be crucial for both the correct diagnosis and for timely efficient treatment of the malignant forms. We used for the first time transgenic (Tg) mice (Sp6; mu/kappa) to study the origin of MG. In the mu, kappa Tg mice a small proportion of B cells can still produce endogenous IgM. These cells are of B-1 cell origin. The MG in Tg mice showed a later onset and a lower frequency than those in littermate control mice, mainly due to a four times lower frequency of benign monoclonal gammopathy. The 10% of B-1 cells that were able to produce endogenous Ig led to the development of MG in a frequency that was half the number of MG found in normal littermates. None of the MG in Tg mice produced an Ig of the Tg origin and therefore it can be concluded that they originated from B-1 cells.

An example of idiotypic mimicry

We have evaluated the impact of transgenic immunoglobulin (TGIg) expression on endogenous antibody repertoires. The transgenic system was chosen as to allow for normal recombination of endogenous Ig genes, secretion of TGIg from early development on, and distinguishing the TGIg from endogenous Ig by several serological markers on the C and V regions of the molecules. The transgenic construct encodes a complete anti-(4-hydroxy-3-iodo-5-nitrophenyl)acetyl (NP) antibody molecule carrying a well-defined idiotype, bearing a lambda 1 light chain and a chimeric heavy chain encoded by a human alpha 2 C region devoid of its membrane exon, and the murine B1.8 VDJ-region. Endogenous antibody repertoires were analyzed in mitogen-driven limiting dilution cultures, in single-cell assays for naturally activated Ig-secreting cells, and in hybridomas derived by direct fusion of spleen cells from unmanipulated animals. The results show that a very high frequency of splenic resting B cells and plasma cells in transgenic animals produce IgM with B1.8-cross-reactive idiotypes. This was confirmed by hybridoma analysis which also established that the levels of transgene expression and of idiotype-positive IgM production by the same cell are not correlated. The affinities of idiotype-positive endogenous Ig varied, but were generally several orders of magnitude lower than the transgene-encoded idiotype. V regions from idiotype-cross-reactive IgM heavy chains showed marked diversity in sequences that were all different from the transgenic B1.8. These results are compatible with idiotypic mimicry resulting from intercellular selection based on degenerate, whole V region reactivities.

Lack of allelic exclusion in B cell chronic lymphocytic leukemia

We determined the immunoglobulin (Ig) V(H) subgroup expressed by the leukemia cells of 108 patients with B cell chronic lymphocytic leukemia (CLL). Surprisingly, we found that six samples (5%) each expressed Ig of more than one V(H) subgroup. Southern blot analysis demonstrated that these samples each had rearrangements involving both Ig heavy chain alleles. Nucleic acid sequence analyses of the Ig cDNA revealed each to express two functional Ig V(H) genes: V(H)3-33 and V(H)4-39; V(H)3-7 and V(H)4-39; V(H)3-23 and V(H)4-61; V(H)2-70 and V(H)3-30.3; or V(H)3-30 and V(H)4-b (DP67). One sample expressed three Ig V(H) genes: V(H)2-70, V(H)3-7, and V(H)4-59. Despite having more than one Ig heavy chain transcript, each sample was found to express only one functional Ig light chain. From the primary sequence, we deduced that the Ig of some of these CLL samples should react with Lc1, a monoclonal antibody (mAb) reactive with a supratypic cross-reactive idiotype present on Ig encoded by a subgroup of Ig V(H)4 genes (namely, V(H)4-39, V(H)4-b [DP-67], V(H)4-59, or V(H)4-61), and B6, an mAb that reacts with Ig encoded by certain Ig V(H)3 genes (namely, V(H)3-23, V(H)3-30, or V(H)3-30.3), and/or modified staphylococcal protein A (SpA), a 45-kilodalton bacterial "superantigen" that reacts with most Ig of the V(H)3 subgroup. Flow cytometric analyses revealed that such samples did in fact react with Lc1 and B6 and/or SpA, but not with control mAbs of irrelevant specificity. This study demonstrates that a subset of CLL patients have leukemic B cells that express more than one functional Ig heavy chain.

Production of anti-phosphorylcholine antibodies of the T15 idiotype in CBA/N xid mice: investigation of the defect using a T15 immunoglobulin transgene

A notable defect in CBA/N xid mice is their relative inability to make antibodies to phosphorylcholine (PC), particularly those of the T15 idiotype which predominate in the anti-PC responses of immunologically normal mice. To investigate the basis of this defect, we introduced functionally rearranged genes encoding a T15+ PC-binding immunoglobulin G antibody into the germline of these animals. Expression of these genes in the xid cells was observed, shown by the existence of a distinct population of T15+ cells (3 x 10(6)) in the spleen of the transgenic animals, and the presence of PC-binding T15+ IgG antibodies (1-15 micrograms/ml) in the serum. Mixed antibody molecules were also found, however, which were composed of both transgene-encoded and endogenously-derived chains. Existence of the T15+ cells in these animals seemed normal, since these were not depleted (to any great extent) and were immunocompetent as well. The latter was shown by the increased T15+ antibody production in the transgenic animals when stimulated with a PC-associated thymus-independent type 1 (TI-1) antigen and anti-idiotype antibodies, but not with the pneumococcal TI-2 antigen. This is similar to the PC-specific (T15-) responsiveness of normal CBA/N xid mice. Based on these results, we argue that a reason why T15+ antibodies are not normally made by CBA/N xid animals is because T15+ genes are not utilized or, as with any T15+ precursors present, selected for in these animals, in contrast to normal mice where the Lyb-5 or CD5 cells (which are absent in CBA/N xid animals) are known to be specially endowed to make such antibodies.

Expression of a MHC class II transgene determines both superantigenicity and susceptibility to mammary tumor virus infection

Milk-borne mouse mammary tumor virus (MMTV) is a type B retrovirus that induces mammary carcinoma. Infectious MMTV, as well as genomically integrated mouse mammary proviruses, encode superantigens that are recognized by T cells that express appropriate T cell receptor V beta products. To determine the relationship between the superantigenic property of milk-borne MMTV and its in vivo infectivity, mice which were either positive or negative for expression of a transgene-encoded E alpha E beta class II major histocompatibility complex (MHC) product were exposed to milk borne C3H MMTV. Superantigen-mediated deletion of V beta 14-expressing T cells occurred only in E alpha transgene-positive mice, indicating that the deletion was E alpha E beta dependent. When mice were analyzed for viral infection by assaying viral p28 in the milk of recipient females, significant p28 levels were found only in E alpha E beta transgene-positive mice. Similarly, the presence of C3H MMTV LTR mRNA in mammary glands, as detected by PCR, paralleled p28 levels. These findings indicate that E alpha expression or the E alpha-dependent T cell response to viral superantigen is causally related to susceptibility to MMTV infection, and that lack of a permissive class II product can protect mice from virus infection.