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.

Internucleosomal chromatin degradation in myeloma and B-hybridoma cell cultures

The activity of Ca/Mg-dependent endonuclease (CME) is strongly inhibited in myeloma X-63.Ag8.653 and B-hybridoma MLC-1c as compared with mouse splenocytes. Nevertheless, pronounced internucleosomal chromatin degradation occurs in both cell lines during long-term cultivation without passing. In isolated cell nuclei of X-63 the activation of CME, which precedes chromatin fragmentation in vivo and loss of cell viability, is revealed. The time-course of CME activation is opposite to cell proliferation and is not accompanied by alterations in enzyme quantity. The results suggest that cell death of X-63 and MLC-1c occurs via apoptosis, and involves the mechanisms controlling the activation and/or interaction of CME with chromatin.

[Use of monoclonal antibodies in comparative studies of Ca, Mg-dependent endonuclease in cell nuclei]

Eight hybridoma cell lines derived from fusion between myeloma X-63 and mouse splenocytes were found to secrete monoclonal antibodies against Ca/Mg-dependent endonuclease of human spleen cell nuclei. Two of them, termed N and S, were used in comparative research of enzymes from different organs and species of animals. The data obtained show that N and S antibodies recognize different antigenic determinants of the enzyme molecule. Cross-reactions of antibodies with different antigens having similar antigenic determinants, exist in Ca/Mg-endonuclease of such species as man, mouse, rat and cattle. The evolutionary conservatism of this enzyme is suggested. The data show that the existence of tissue-specific (thymus-specific and spleen-specific) isoforms of Ca/Mg-endonuclease of cell nuclei is possible.

[Dynamics of the endo-DNAase activity of cell nuclei of mouse thymus and spleen lymphocytes during the immune response]

Endo-DNAse (mostly Ca/Mg-dependent endonuclease) activity was studied in extracts of lymphocyte cellular nuclei from the spleen and thymus of mice upon their immunization with sheep red blood cells. Endo-DNAses were detected by their action on super-stranded DNA pBR 322. It has been established that endo-DNAse activity considerably changes in the course of immune response. The changes start in the early (induction) phase of immune response, are characterized by certain regularities and are distinct in thymus and spleen lymphocytes. It is assumed that endo-DNAses of lymphocyte cellular nuclei are involved in antigen-dependent lymphocyte differentiation.