Isotype switching in anti-immunoglobulin-activated B lymphoblasts: differential requirements for interleukin 4 and other lymphokines to elicit membrane vs. secreted IgG1

Analysis of cellular phenotype during in vitro immunization of murine splenocytes for generating antigen-specific immunoglobulin

Although various in vitro immunization methods to generate antigen-specific antibodies have been described, a highly effective method that can generate high-affinity immunoglobulins has not yet been reported. Herein, we analyzed a cellular phenotype during in vitro immunization of murine splenocytes for generating antigen-specific immunoglobulins. We identified a combination of T cell-dependent stimuli (IL-4, IL-5, anti-CD38 and anti-CD40 antibodies) plus lipopolysaccharides (LPS) that stimulates antigen-exposed splenocytes in vitro, followed by induction of the cells phenotypically equivalent to germinal center B cells. We also observed that LPS induced high expression levels of mRNA for activation-induced cytidine deaminase. We stimulated antigen-exposed splenocytes, followed by the accumulation of mutations in immunoglobulin genes. From the immunized splenocytes, hybridoma clones secreting antigen-specific immunoglobulins were obtained.

Divide and conquer: the importance of cell division in regulating B-cell responses

Proliferation is an essential characteristic of clonal selection and is required for the expansion of antigen reactive clones leading to the development of antibody of different isotypes and memory cells. New data for mouse and human B cells point to an important role for division in regulating isotype class and in optimizing development of protective immunity by the regulated entry of cells to the plasma cell lineage.

Isotype switching by human B cells is division-associated and regulated by cytokines

Isotype switching by murine B cells follows a pattern whereby the proportion of cells undergoing switching increases with division number and is regulated by cytokines. Here we explored whether human B cells behaved in a similar manner. The effect of IL-4, IL-10, and IL-13, alone or in combination, on Ig isotype switching by highly purified naive human CD40 ligand (CD40L)-activated B cells was measured against division number over various harvest times. Switching to IgG was induced by IL-4 and, to a lesser extent, IL-13 and IL-10. The combination of IL-10 with IL-4, but not IL-13, induced a higher percentage of cells to undergo switching. Isotype switching to IgG by human CD40L-activated naive B cells was found to be linked to the division history of the cells: IgG(+) cells appeared in cultures of B cells stimulated with CD40L and IL-4 after approximately the third cell division, with the majority expressing IgG1, thus revealing a predictable pattern of IgG isotype switching. These results reveal a useful quantitative framework for monitoring the effects of cytokines on proliferation and isotype switching that should prove valuable for screening Ig immunodeficiencies and polymorphisms in the population for a better understanding of the regulation of human humoral immune responses.

CD9 is a unique marker for marginal zone B cells, B1 cells, and plasma cells in mice

Marginal zone (MZ), follicular (FO), and B1 B cells form the long-lived naive B cell compartment. To identify surface markers that define MZ B cells in mice, we generated a panel of mAbs reactive with MZ but not FO B cells. One of these mAbs, MZ3, was found to recognize the tetraspanin CD9. CD9 expression not only distinguishes MZ B cells from FO B cells but also divided peritoneal cavity B1 cells into smaller subsets. After short-term in vitro stimulation with various mitogens, FO B cells failed to induce CD9 protein, while MZ B cells up-regulated the level of CD9 protein. However, after prolonged culture of FO B cells with LPS, surface CD9 was induced, together with syndecan 1, indicative of plasma cell differentiation. Following immunization with a T-independent-2 Ag, R36A, or a T-dependent Ag, SRBC, we found that CD9 is not expressed by germinal center B cells but is eventually expressed on plasma cells in response to both T-independent-2 and T-dependent Ags. Collectively, these results suggest that MZ B cells and B1 cell subsets are the immediate precursors of plasma cells in the primary response and that CD9 is acquired by T-dependent plasma cells.

Cell division number regulates IgG1 and IgE switching of B cells following stimulation by CD40 ligand and IL-4

CD40 ligand (CD40L) and IL-4 are sufficient to induce resting murine B cells to divide and switch isotypes from IgM and IgD to IgG1 and IgE. Tracking of cell division following (5- and 6) carboxyfluorescein diacetate succinimidyl ester (CFSE) labeling revealed that B cells expressed IgG1 after three cell divisions, and IgE after five. The probability of isotype switching at each division was independent of both time after stimulation and of the dose of CD40L. IL-4 concentration regulated the number of divisions that preceded isotype switching. Loss of surface IgM and IgD was also related to cell division and appeared to be differentially regulated. B cell proliferation was typically asynchronous with the proportion of cells in consecutive divisions being markedly affected by the concentration of CD40L and IL-4. Simultaneous (5-bromo)-2'-deoxyuridine labeling and CFSE staining revealed that B cells in each division cycle were dividing at the same rate. Therefore, division cycle asynchrony resulted from dose-dependent variation in the time taken to enter the first division cycle. These results suggest that T-dependent B cell expansion is linked to predictable functional changes that may, in part, explain why IgE is produced in response to prolonged antigenic stimulation.

Secondary IgE responses in vivo are predominantly generated via gamma 1 epsilon-double positive B cells

We have recently developed a model in which mice were treated with IL-4 after primary immunization, resulting in elevated total serum IgG1 and IgE levels, but decreased antigen-specific levels and memory formation for these isotypes. In this report, we describe that these effects of IL-4 are mediated at the B cell and not the T-cell level. Major changes occurred in the gamma 1 epsilon-double positive B-cell population which is increased as a result of IL-4 treatment. Moreover, it is shown that gamma 1 epsilon-double positive B cells can develop in vitro out of gamma 1-positive primed B cells and that these double positive cells can differentiate into IgG1- and IgE-secreting cells. The existence of gamma 1 epsilon-double positive memory B cells can explain the differences in cytokine dependence of TNP-specific memory IgG1 and IgE responses found after adoptively transferring primed spleen cells into irradiated naive recipients. Whereas the IL-4 independent TNP-specific memory IgG1 responses could be blocked efficiently by neutralizing IL-5 and IL-6, TNP-specific memory IgE responses were virtually not susceptible to such treatment. These IgE responses were also not susceptible to IFN-gamma, used in doses that could inhibit the primary IgE response. Inhibition of the TNP-specific memory IgG1 response by neutralizing IL-5 and IL-6 is accompanied by a 10-fold increase of the IL-4 independent TNP-specific IgE memory response. These data indicate that secondary IgE responses primarily result from B cells that are either switched to IgG1, or are double positive for IgG1 and IgE, thereby suggesting a minor role for epsilon-single positive B cells in secondary IgE responses.

Suppression of polyclonal and antigen-specific murine IgG1 but not IgE responses by neutralizing interleukin-6 in vivo

The crucial role of interleukin (IL)-4 in the induction of murine IgG1 and IgE responses, which are coupled through the process of sequential isotype switching, has been well documented. Whereas IL-4 is obligatory for the induction of IgE responses, it enhances IgG1 responses. In this study, using neutralizing antibodies, we provide evidence that, besides IL-4, also IL-6 is required for obtaining peak IgG1 responses. The mRNA levels of these two cytokines are coordinately expressed in the spleen of mice immunized with trinitrophenol-keyhole limpet hemocyanin (TNP-KLH). No IL-6 requirement was observed for peak IgE responses. The IL-6 dependence of IgG1 responses was found for both antigen-specific and polyclonal responses. Moreover, it was noted using TNP-KLH and goat anti-mouse (GAM) IgD as antigen that polyclonal IgG1 responses are more dependent on IL-6 than antigen-specific responses. In vitro experiments revealed that exogenous IL-6 neither enhanced nor inhibited the IgG1 and IgE production by naive B cells, suggesting that IL-6 did not interfere with the IL-4-induced isotype switch potential. Primary and memory IgG1 responses were both similarly dependent on IL-6. These observations point to a role of IL-6 in the terminal differentiation of B cells switched to IgG1. Neutralization of IL-6 did not inhibit either antigen-specific or polyclonal IgE responses. Therefore, it was concluded that IL-6 is not involved in the terminal differentiation of B cells switched to IgE. These findings thus provide a distinct role for IL-6, besides IL-4, in regulating murine IgG1 responses. The formation of IgE, however, is completely dependent on IL-4 alone.

The biology of interleukin-5 and its receptor

IL-5 is primarily a T-cell-derived cytokine that has multiple regulatory functions on eosinophils and (in the mouse) on antibody-secreting B cells. A complex network of cytokines appear to control transcription of the gene for IL-5 and its production. Abnormally high levels of this cytokine are associated with infections with tissue-dwelling parasites and a diverse group of hypereosinophilic conditions of no known etiology. Our understanding of the biological role of IL-5 in the regulation of Ig production and the development of immunity to parasites is far from complete, but basic knowledge of its action at the cellular level is accumulating and will be critical for the intelligent application of immunotherapy with IL-5 or antibodies to IL-5 in infectious, neoplastic, and possibly other diseases.

Increased production of antigen-specific B lymphocytes during in vitro immunization using carrier-specific T helper hybridomas

An in vitro method to increase the production of hapten-specific antibody-forming B cells (AFC) using a carrier-specific T helper hybridoma and murine splenocytes is described. Naive splenocytes (6 x 10(6)/ml) are cultured in vitro in the presence of a hapten-carrier conjugate (DNP.OVA) and OVA-specific T helper hybridomas (0.5 x 10(6)/ml). After 4-5 days in vitro immunization (IVI), the maximum number of DNP-specific AFC were found using a spot-ELISA with twice the number of IgM positive cells as IgG positive AFC. The presence of antigen in the form of a hapten-carrier complex and the use of a carrier-specific Th hybridoma resulted in more hapten-specific AFC than when neither antigen nor Th hybridoma were present or when antigen alone or T help alone were used. Also when the hapten was conjugated to a carrier not recognised by the carrier-specific Th hybridoma there were considerably fewer (less than 50%) hapten specific AFC formed. When in vivo primed splenocytes (DNP) were boosted in vitro (IVB) under the same conditions as for IVI most hapten-specific AFC were found on day 4 and both anti-DNP IgM and IgG AFC were increased relative to IVI. Again most AFC were found when hapten was bound to the relevant carrier. In conclusion, carrier-specific T hybridomas can be used in an in vitro immunization procedure with naive or primed splenocytes to increase the frequency of anti-hapten AFC. This method offers an improvement over the current in vitro immunization procedures for the production of monoclonal antibodies.

Interleukin 5 (IL-5) provides a signal that is required in addition to IL-4 for isotype switching to immunoglobulin (Ig) G1 and IgE

We have examined the contributions of Interleukin 4 (IL-4), IL-5, and other stimuli to the expression of Immunoglobulin G1 (IgG1) and IgE in murine B lymphoblasts activated with anti-Ig. The combination of IL-4 and -5 induced B lymphoblasts to proliferate and to secrete IgM and IgG1. However, an additional stimulus was required along with IL-4 and -5 for induction of IgE secretion. This stimulus was provided by lipopolysaccharides (LPS) or cytokines produced by TC-1 or EL4 cells. In the absence of IL-5, exceptionally high concentrations of IL-4 (greater than 1,000 U/ml) were required to elicit IgG1 and IgE secretion from B lymphoblasts cultured with either LPS or TC-1-conditioned media (CM). To investigate regulation of expression of gamma 1 and epsilon genes by IL-4, -5, and LPS, the requirements for induction of gamma 1 and epsilon germline and productive transcripts were examined. Germline gamma 1, but not epsilon, transcripts were detected in RNA from B lymphoblasts treated with IL-4 and -5 for 48 h. In contrast, both germline gamma 1 and epsilon transcripts could be detected in B lymphoblasts cultured with IL-4 and LPS, and steady state levels of germline gamma 1 transcripts were four- to sevenfold higher in blasts cultured with LPS and IL-4, compared with blasts cultured with IL-4 and -5. LPS enhanced steady state levels of germline transcripts induced by IL-4, but LPS did not promote substantial accumulation of productive gamma 1 and epsilon transcripts. In contrast, IL-5 did not affect steady state levels of germline transcripts stimulated by IL-4, but did markedly increase levels of productive gamma 1 and epsilon transcripts. Thus, lymphokines regulate two distinct events in isotype switching: induction of germline transcripts (IL-4), and production of VDJ-C gamma 1 and VDJ-C epsilon mRNA (IL-5), which leads to secretion of IgG1 and IgE.