Direct evidence for a primary immune response of murine B-lymphocytes after in vitro immunization of dissociated splenocytes

Influence of antigen presentation and exogenous cytokine activity during in vitro primary immunizations employed for the generation of monoclonal antibodies

Hybridomas secreting monoclonal antibodies (MAbs) against African horse sickness virus (AHSV) were generated using different AHSV antigen preparations (inactivated AHSV, semi-purified virus, and a preparation of nonstructural viral proteins) in one of three different in vitro primary immunization systems: (i) the Cel-prime kit, a method using immunization of splenocytes aided by antigen-primed support cells; (ii) a system based on a cytokine soup derived from a mixed lymphocyte reaction plus stimulated EL4-IL-2 cells; (iii) a system based on a cytokine soup derived from splenocytes stimulated by pokeweed mitogen in order to obtain a mixture of cytokines enriched for Th2 lymphokines. The viability of immunized BALB/c mouse splenocytes, immunoglobulin production by the subsequently generated hybridomas, and the specificity of the MAbs were compared. The most efficient in vitro primary immunization system was the Cel-prime system employing semi-purified antigen. This efficiency was manifest in terms of a greater viability of the splenocytes in the immunization, as well as a higher number of specific antibody-secreting hybridomas. It seems probable that the support cells of the Cel-prime system have an accessory function such as that attributed to antigen-presenting cells. Such a function would result in impairment of apoptosis, and thus increase the viability of the splenocytes in the in vitro primary immunization system, as well as enhancing stimulation of the immune response against the antigen used. The presence of cytokines at the beginning of the in vitro primary immunization did have an influence, but this was secondary to what appeared to be the major event of cellular interaction associated with the accessory cell function of the support cells.

Generation and characterization of putative monoclonal and polyclonal antibodies against tartrate-resistant acid phosphatase

The generation of monoclonal antibodies (MAbs) specific for tartrate-resistant acid phosphatase (TRAP) may aid development of a serological immunoassay for this marker of bone resorption. The lack of MAbs to TRAP largely reflects the difficulty in obtaining sufficient antigen for in vivo immunization strategies. We have circumvented this problem by using in vitro immunization, requiring a small amount of TRAP isolated from osteoclastoma tumor. Two MAbs designated 312D and 310A were generated that exhibited weak anti-TRAP activity in enzyme-linked immunosorbent assay and immunocytochemistry. A polyclonal antibody to TRAP (Ab8023) was also raised in rabbit, using synthetic peptide. Ab8023 and MAbs 310A and 312D exhibited no activity against TRAP in dot-blotting experiments. Further characterization in enzyme-liked immunosorbent assay showed that Ab8023 was remarkably specific for TRAP whereas MAb 312D cross-reacted with another metalloenzyme, human prostatic acid phosphatase.

Rapid generation of monoclonal antibody-secreting hybridomas against African horse sickness virus by in vitro immunization and the fusion/cloning technique

Splenocytes from non-immune mice were stimulated in vitro using an equimolar mixture of factors from mixed lymphocyte reaction (MLR) and from phorbol-12-myristate 13-acetate (PMA) stimulated EL-4 cells, and concomitantly immunized with inactivated African horse sickness virus (AHSV) antigen serotype 4 or viral proteins 2 and 5 from AHSV serotype 9. Fusion with NSO myeloma cells was performed five days after primary or secondary stimulation/immunization. The record of hybridoma growth after a standard method of fusion, expansion of cells and subsequent cloning was compared with a fusion/cloning method in which cells were cloned within 2 to 3 days of the fusion event. Detection of antigen specific antibodies in the hybridoma culture supernatants was successful only with cells derived from primary stimulation/immunizations. Antibodies were detected using an indirect ELISA with the immunizing antigen coated on to the surface of the plates. Monoclonal hybridomas were isolated within 2 to 3 weeks using the fusion/cloning method, compared with the standard method, where it took 4 to 5 weeks. Although the total number of clones isolated from the fusion/cloning method was less than that obtained through the standard method, the yield of specific antibody-producing hybridomas as a percentage of the total picked was often more efficient with the fusion/cloning method. With respect to the immunoglobulin isotype produced, not all of the antibodies could be classified by the ELISA system used; 14% of anti-AHSV positive clones were identified as IgG-secreting cells, 25% as IgM-secreting, 18% were cross-reacting with IgG and IgM, and 43% could not be classified. Similar results in all aspects of the work were obtained whether a crude infected cell extract or purified outer capsid polypeptides VP2/5, from serotype 4 and serotype 9 respectively, were used.

Production of epitope specific monoclonal IgG antibodies to HLA class II molecules by combining in vivo and in vitro immunization

To study the expression of HLA-DQ beta chain alleles associated with type 1 diabetes, mAbs were generated from mice immunized with synthetic peptides representing allelic HLA-DQw7 and HLA-DQw8 beta chain sequences. The splenocytes from immunized mice were fused with myeloma cells, either immediately after or following additional in vitro boosting with peptide. Peptide-specific mAbs, predominantly of the IgG isotype, were isolated only from in vitro boosted splenocytes. Immunoblot analysis showed that several of the mAbs cross-reacted with DQ beta chain molecules. One mAb to a peptide representing DQw8 beta position [49-60] specifically recognised the DQw8 beta chain. Three mAbs to a peptide representing DQw8 beta position [39-52] specifically recognised an epitope consisting of Gly-Val-Tyr in position 45-47, i.e., all DQ beta alleles except DQw7 beta (position 45-47: Glu-Val-Tyr) and DQw2 beta (position 45-47; Gly-Glu-Phe). In FACS analysis these mAbs bound lymphocytes with the same specificity as found by immunoblotting analysis. Thus, by combining in vivo and in vitro immunization we have generated a number of epitope specific monoclonal IgG antibodies that distinguish closely related HLA-DQ beta chain alleles in predetermined positions.

Production of monoclonal antibodies specific for African swine fever virus following in vitro primary immunization of mouse splenocytes in the presence of stimulated T lymphocyte supernatants

Splenocytes from non-immune mice were stimulated in vitro using a kit of cytokine preparations (obtained from murine MLR and EL-4 cell cultures), and concomitantly immunized with African swine fever (ASF) virus antigen. In addition, fusions were performed at 5 days after primary or secondary stimulation/immunization. The detection of specific antibodies in the culture supernatants was not successful. In contrast, specific antibody-producing hybridomas could be generated, and this was at least comparable to a standard in vivo immunization regime, even though the optimum fusion ratio employed with these in vitro immunized splenocytes was one which is not optimum when in vivo immunized lymphocytes are used. Consequently, it would appear that hybridoma generation is a more sensitive method than the direct measurement of antibody at detecting in vitro primary immune responses. After primary in vitro immunization, the majority of immunoglobulins produced were apparently of the IgG isotype, with only 8-17% clearly IgM. These antibodies were mainly against VP73 (the major viral envelope protein) as expressed on viral antigen extracted from infected cells, although other specificities were also found. This demonstrated, by in vitro means, that the VP73 carried dominant immunogenic epitopes on ASF virus. Such observations show that the in vitro responses were closely related to those which have been detected in vivo.

High yields of specific hybridomas obtained by electrofusion of murine lymphocytes immunized in vivo or in vitro

We have studied the use of electrofusion to obtain hybridomas producing antigen-specific antibodies after immunization of murine lymphocytes in vitro. Under optimal conditions fusion frequencies of the order of magnitude of 10(-3) were obtained, which is approximately 80-fold higher than the mean value obtained with fusion induced by polyethylene glycol. The number of antigen-specific hybridomas was also increased in a comparable way. The high yields of specific hybridomas observed with electrofusion were independent of the immunization procedure, the antigen or the hapten of interest, or the sources of the lymphocytes. The data presented in this paper indicate that electrofusion may be an extremely attractive alternative method for immortalization of human lymphocytes following immunization in vitro.

Optimal conditions for the generation of monoclonal antibodies using primary immunisation of mouse splenocytes in vitro under serum-free conditions

We have further optimised the serum-free in vitro immunisation system described by Ossendorp et al. (J. Immunol. Methods 91, 257, 1986) for the generation of hybridomas secreting specific antibodies. Thyroglobulin and the hapten 2-phenyl-5-oxazolone coupled to chicken serum albumin were used as antigens. For an optimal outgrowth of antigen-specific B cells the presence of T cells, thymocyte-conditioned medium and antigen are required. The addition of supernatant from EL-4 cells (stimulated by phorbolmyristate acetate) inhibits the outgrowth of antigen-specific B cells. Using six-well plates with a surface area of 10 cm2 per well, an optimal IgM response was obtained when 10(7) splenocytes in a total volume of 2 ml/well were cultured for 3 days in the presence of antigen and thymocyte-conditioned medium. Increasing the concentration of cells whilst maintaining a constant surface area resulted in a decreased response.

Requirements for the generation of memory B cells in vivo and their subsequent activation in vitro for the production of antigen-specific hybridomas

We have studied the conditions required for the activation in vitro of memory B cells generated in vivo. BALB/c mice were immunised by a single injection of antigen emulsified in Freund's complete adjuvant. Splenocytes were isolated after different time intervals and cultured in a serum-free medium in the presence of antigen and thymocyte-conditioned medium. After 3 days the splenocytes were fused with myeloma cells. A minimum time interval of more than 2 weeks between priming in vivo and stimulation in vitro was required in order to obtain antigen-specific IgG-secreting hybridomas. After a time interval of 4 weeks or longer most of the antigen-specific hybridomas secreted IgGs. During stimulation in vitro the presence of antigen and of T cells was found to be essential for obtaining an antigen-specific IgG response. The addition of thymocyte-conditioned medium enhanced the IgG response.