SCID-hu-PBL: a model for making human antibodies?

The ability of SCID mice to accept xenografts has been utilized in the search for a laboratory model that mimics the human immune system. Intraperitoneal injection of human peripheral blood lymphocytes into SCID mice constitutes a feasible and easy way of establishing an experimental immune system that may be manipulated without the concern that applies to human volunteers. Of particular interest to the immunotechnologists is the possibility of utilizing SCID-human chimeras to generate a human affinity maturated IgG response, which could form the basis for the development of therapeutic antibodies. This review discusses some of the potential limitations of SCID-hu-PBL chimeras in achieving this goal.

Enhancement of specific immunoglobulin production in SCID-hu-PBL mice after in vitro priming of human B cells with superantigen.

Priming of human mature B cells in vitro with staphylococcal enterotoxin A (SEA) prior to transplantation of the B cells into severe combined immunodeficiency (SCID) mice, together with human T-helper cells, resulted in higher and more uniform concentrations of serum IgG in the mice. This indicated that a large number of B cells had become activated, which was supported by the finding that SEA priming resulted in production of immunoglobulin displaying a more normal kappa/lambda ratio than was obtained in the absence of SEA priming. However, IgM concentrations were not affected by SEA priming. Immunization of mice, transplanted with SEA-primed B cells, with both primary and secondary antigens resulted in a high specific IgG response to both types of antigen. The elevated levels of specific antibodies were not merely the consequence of an unspecific stimulation of B cells caused by SEA, as the ratio of specific antibody to total IgG was much higher in animals receiving SEA-primed B cells. Thus, a co-operative effect on immunoglobulin production of stimulating B cells via surface immunoglobulin and help delivered by SEA-activated T-helper cells was indicated. A specific antigen-dependent IgM response to a secondary antigen was observed as well, but was, in contrast to the IgG response, not influenced by SEA priming of B cells. No IgM antibodies with reactivities to the primary antigens were detected in the SCID sera at any time-point after immunization. The results thus indicate that SEA might replace T-cell epitopes in antigens and efficiently recruit an abundance of T-cell help to B cells, resulting in enhanced production of specific IgG antibodies.

Antigen-specific activation of B cells in vitro after recruitment of T cell help with superantigen.

BACKGROUND: Human B cells can proliferate in vitro after stimulation with anti-Ig and via the CD40 molecule. Superantigens like SEA which bind to MHC class II antigens on, e.g. B cells can polyclonally activate T cells via interaction with their TcR. The activated T cell subsequently activates the B cells to proliferation and Ig-production. OBJECTIVES: To investigate whether superantigen could be used to direct polyclonal T cell help to human B cells stimulated by antigen in a restricted manner resulting in production of antigen-specific antibodies in vitro. STUDY DESIGN: Purified B cells were preincubated with the antigen in manners allowing crosslinking of surface-Ig. The antigen exposed B cells were then cultured together with autologous CD4+ helper T cells and in the presence of various concentrations of SEA. Antibody production was measured by ELISA after 7-12 days of culture. RESULTS: Antigen-specific activation of B cells could be obtained after stimulating the B cells with antigen or anti-surface-Ig antibodies in the presence of T helper cells and SEA. The degree of B cell activation (proliferation as well as antibody production) depended on the dose of antigen as well as on the dose of SEA used. Increased crosslinking of surface-Ig on antigen-specific B cells enhanced Ig production. Specific antibody production to a secondary recall antigen (tetanus toxoid) and to primary antigens (DNP and GM2) were obtained. The specific B cell response was dependent on contact between T and B cells. CONCLUSION: the results obtained demonstrate that the superantigen SEA can recruit T cell help to human B cells specifically stimulated by antigens, resulting in production of antigen reactive antibodies in vitro.

Induction of primary antigen-specific immune reponses in SCID-hu-PBL by coupled T-B epitopes.

Adoptive transfer of human lymphoid cells into immunodeficient (SCID) mice lacking the ability to functionally rearrange T- and B-cell receptor genes constitutes a unique model to study and manipulate human immunocytes. We have investigated this model for the purpose of generating an antigen-specific primary humoral immune response. Peripheral blood lymphocytes (PBL) derived from blood donors were used to repopulate SCID mice, which subsequently were immunized with different B-cell epitopes coupled to either tetanus toxoid (TT), or to a promiscuous helper epitope of TT, or by incorporating the antigens into a liposome construct. By recruiting the necessary T-cell help found in the T-cell memory compartment against TT, primary immune responses were obtained against the hapten dinitrophenyl (DNP), the V3 loop peptide derived from glycoprotein (gp120) (HIV-1), the melanoma-associated GD2 ganglioside and ovine submaxillary mucin. The primary immune response against the GD2 ganglioside was induced by incapsulating TT into GD2-containing liposomes. These liposome constructs also allowed us to induce a high human IgG serotitre (3000-4000) against this normally not very immunogenic ganglioside.

Antigen-specific primary immune response of human B-lymphocytes after in vitro immunization with GM3 ganglioside.

In vitro immunization of human B-lymphocytes was performed with liposomes containing the monosialoganglioside GM3, with or without either complete tetanus toxoid or a synthetic T helper epitope derived from tetanus toxin (determinant 830-843). The immunized B-cells were Epstein-Barr virus transformed and the human anti-ganglioside antibody response was evaluated using an indirect ELISA against different mono- and disialogangliosides. Clones producing antigen-specific human antibodies of the IgM isotype against the ganglioside GM3 used as the immunogen were selected and one clone, IM-11, was further characterized. In addition, a method of positive selection using GM3-coated magnetic beads has been developed which allowed us to rescue unstable clones. The binding of the human antibody IM-11 to a large panel of glycosphingolipids separated on thin-layer plates was studied. The human MAb IM-11 was found to bind strongly to NeuAcGM3, IV3 NeuAcnLc4 and sulfate containing glycosphingolipids and weakly to NeuGcGM3. Immunohistological staining of melanoma and breast cancer biopsy sections showed a selective reactivity of IM-11 with tumor cells which varied among different tumors.

Human Th0-type T helper-cell clone supports antigen-specific immunoglobulin production in scid/beige-hu mice.

Tetanus toxoid-specific T cells have been generated from human splenic lymphocytes by an initial 6-day stimulation period with antigen, followed by a proliferation period with recombinant IL-2 and human feeder cells. Proliferating T cells were subsequently cloned by limiting dilution. A human T-cell clone that was functionally characterized showed: (i) a specific proliferative response to tetanus toxoid in the presence of autologous Epstein-Barr virus (EBV)-transformed lymphoblastoid cells; (ii) a phenotype characteristic for the helper/inducer CD4+/CD8-/CD450R0+ T cells, and (iii) a lymphokine profile, as determined by mRNA analysis, representative of Th0-like human CD4+ T helper cells. This tetanus toxoid-specific T-cell clone which showed antigen-dependent helper activity for antibody production by autologous B cells in vitro could also provide T-cell help to antigen-specific human B cells transplanted into severe combined immunodeficiency (scid/beige) mice.

Effect of cell-derived growth factors and cytokines on the clonal outgrowth of EBV-infected B cells and established lymphoblastoid cell lines.

Epstein-Barr virus (EBV) is a potent inducer of polyclonal B lymphocyte proliferation and is widely used as a tool for the establishment of B cell lines producing human monoclonal antibodies. However, because of low transformability, low clonability, and the inherent instability of EBV-infected B cells, valuable antibody-producing B cells are often lost during this procedure. We have here examined various cell-derived cytokines for their ability to enhance both the cellular outgrowth of newly infected B cells and the clonability of infected B cells and lymphoblastoid cell lines. Our results show that the murine thymoma cell line EL-4 is superior to peripheral blood mononuclear cells in both cellular outgrowth and cloning experiments, whereas monocyte-derived factors and monocyte cell lines were less capable than peripheral blood mononuclear cells in enhancing cellular outgrowth and cloning. Furthermore, the human T cell hybridoma cell line MP6 that secretes a B cell growth and differentiation factor, recently identified as an isoform of thioredoxin, is also capable of stimulating EBV-infected B cells and lymphoblastoid cell lines. Co-cultivation of EBV-infected B cells with MP6 cells significantly enhanced the cloning efficiency at the 1 cell/well level. The present results also suggest that one potential role of the MP6-derived thioredoxin could be the up regulation of IL-6 receptor expression in EBV-infected B cells.