Functional heterogeneity of nonresting B cells in human blood

Dual effects of interleukin 4 on antigen-activated human B cells: induction of proliferation and inhibition of interleukin 2-dependent differentiation

We analyzed the effects of interleukin 2 (IL 2) and IL 4 isolated and in association on the specific response of human B cells triggered by trinitrophenylated polyacrylamide beads (TNP-PAA). IL 2 induced an increase (more than 10 times) in the number of hapten-binding cells [detected by a rosette-forming cell (RFC) assay] as well as the generation of antibody-producing cells [detected by a plaque-forming cell (PFC) assay]. IL 4 induced an isolated RFC response without PFC response. We verified that the IL 4 (as well as 12)-induced RFC were hapten specific and mediated through membrane IgM. Density fractionation experiments showed that IL 4-induced RFC were equally distributed between high-density and intermediate-density B cells. IL 2 appeared to drive more B cells into the intermediate density fraction. IL 2-induced PFC belonged to the RFC population and were intermediate-size B cells. IL 2 drove more RFC into an activated stage and it induced the differentiation of a number of them into antibody-producing cells. The evaluation of the proportion of RFC able to incorporate thymidine showed that both IL induced a substantial proliferation of antigen-activated B cells. However, IL 4 inhibited the IL 2-dependent PFC without affecting the number of RFC nor the proportion of proliferating RFC induced by this IL. These results directly demonstrate that human IL 4 triggers the expansion of antigen-activated B cells and selectively inhibits the IL 2-induced differentiation.

Differential mechanism for differentiation into immunoglobulin-secreting cells in human resting B lymphocyte subsets isolated on the basis of cell density

We have investigated differential mechanism for differentiation of human peripheral blood resting B cells to Ig-secreting cells. Purified resting B cells were further fractionated into subsets by discontinuous density gradients of Percoll, and proliferation and differentiation responses to Staphylococcus aureus Cowan I (SAC) and/or T cell-derived soluble factors were studied. High density resting B cells were stimulated to proliferate vigorously in response to SAC, but were poorly differentiated by SAC in presence of T cell factors. In contrast, low density resting B cells failed to proliferate in response to SAC and/or T cell factors; these cells could, however, be induced by stimulation with SAC plus T cell factors to become cells actively secreting Ig. These results indicate that there may exist heterogeneity in the human resting B cells: one subset of resting B cells (B cells with low density) can differentiate directly into Ig-secreting cells without the need for proliferation, and another subset (B cells with high density) can proliferate actively without subsequent differentiation into Ig-secreting cells. To address whether these resting B cell subsets belong to the same lineage, only high density B cells recovered from circulating resting B cells were first stimulated for 7 d with SAC, refractionated on Percoll gradients, and differentiation response of the refractionated B cells to SAC and T cell factors was examined. B cells shifting toward low density fraction were located in the resting status and could differentiate in response to SAC plus T cell factors. These results indicate that some of B cells with high density belong to the same cell lineage as those with low density and they must first proliferate before differentiation.

Functional characterization of human B cells carrying the lymphocyte large sialoglycoprotein gp150

Human B cell-enriched populations were prepared from buffy coats of healthy donors. By means of affinity chromatography, the B cells were separated into two fractions, one enriched in and the other depleted of cells expressing gp150, the large sialoglycoprotein of lymphocytes. In the presence of autologous T cells, monocytes and pokeweed mitogen B cell populations enriched for gp150+ cells gave rise to significantly more plasma cells (cIg+ cells) and secreted significantly more IgG than gp150-depleted populations. In contrast, more or an equal amount of IgM was secreted in cultures containing gp150-depleted cells. The differences between the fractions could not be ascribed to uneven distribution of T3+ cells, OKM1+ cells or B1+ (CD20) cells. However, the gp150-enriched population contained significantly more B2+ (CD21) cells than the gp150-depleted population. These results suggest that the gp150+ B cells differ from gp150- B cells, not only in their responsiveness to T cell differentiation signals but also in their commitment to Ig heavy chain isotype secretion.

Limiting dilution assay for human B cells based on their activation by mutant EL4 thymoma cells: total and antimalaria responder B cell frequencies

The analysis of human B cell responses at the clonal level (limiting dilution assay) is still technically difficult. In the present study we report on a culture system that leads to activation, proliferation and differentiation into antibody-secreting cells (ASC) of about 90% of B cells from peripheral blood or spleen. In this system, B cells are cultured in the presence of a mutant subclone of the mouse thymoma EL4 for B cell activation and human T cell plus macrophage supernatant as source of proliferation and differentiation factors. ASC precursors generating clonal responses of IgM only, IgM plus IgG, or IgG only occurred at a ratio of about 6:3:1. The mean clone size was 380 cytoplasmic Ig+ cells; the mean amount of Ig secreted per clone was 20 ng. Furthermore, it has been found using this system that a considerable proportion of peripheral blood B cells from individuals with a history of malaria infection could generate clones of anti-malaria (Plasmodium falciparum) ASC (range of 0.1 to 1%, n = 6). In a control group of blood donors the corresponding frequencies were 10 times lower (range of 0.01 to 0.1%, n = 9). These results show that the EL4 culture system can be applied to the investigation of the human B cell specificity repertoire and of priming effects such as result from infectious disease.