Relationship between pre-existing viral reservoirs and the re-emergence of plasma viremia after discontinuation of highly active anti-retroviral therapy.

We examined the pathogenic significance of the latent viral reservoir in the resting CD4+ T cell compartment of HIV-1-infected individuals as well as its involvement in the rebound of plasma viremia after discontinuation of highly active anti-retroviral therapy (HAART). Using heteroduplex mobility and tracking assays, we show that the detectable pool of latently infected, resting CD4+ T cells does not account entirely for the early rebounding plasma HIV in infected individuals in whom HAART has been discontinued. In the majority of patients examined, the rebounding plasma virus was genetically distinct from both the cell-associated HIV RNA and the replication-competent virus within the detectable pool of latently infected, resting CD4 + T cells. These results indicate the existence of other persistent HIV reservoirs that could prompt rapid emergence of plasma viremia after cessation of HAART and underscore the necessity to develop therapies directed toward such populations of infected cells.

Protection of rhesus macaques against disease progression from pathogenic SHIV-89.6PD by vaccination with phage-displayed HIV-1 epitopes.

The antigenic polymorphism of HIV-1 is a major obstacle in developing an effective vaccine. Accordingly, we screened random peptide libraries (RPLs) displayed on phage with antibodies from HIV-infected individuals and identified an array of HIV-specific epitopes that behave as antigenic mimics of conformational epitopes of gp120 and gp41 proteins. We report that the selected epitopes are shared by a collection of HIV-1 isolates of clades A-F. The phage-borne epitopes are immunogenic in rhesus macaques, where they elicit envelope-specific antibody responses. Upon intravenous challenge with 60 MID50 of pathogenic SHIV-89.6PD, all monkeys became infected; however, in contrast to the naive and mock-immunized monkeys, four of five mimotope-immunized monkeys experienced lower levels of peak viremia, followed by viral set points of undetectable or transient levels of viremia and a mild decline of CD4+ T cells, and were protected from progression to AIDS-like illness. These results provide a new approach to the design of broadly protective HIV-1 vaccines.

Evolutionary pattern of human immunodeficiency virus (HIV) replication and distribution in lymph nodes following primary infection: implications for antiviral therapy.

Evolutionary patterns of virus replication and distribution in lymphoid tissue during the early phases of HIV infection have not been delineated. Lymph node (LN) biopsies were excised from patients at different times after the estimated time of primary infection. Within 3 months of the acute viral syndrome, HIV was mostly present in individual virus-expressing cells in LNs; trapping of virions in the follicular dendritic cell (FDC) network was minimal or absent, but was the predominant form of HIV detected in LNs of subjects with chronic infection, either recent (4-20 months after primary infection) or long-term (>2-3 years after primary infection). Plasma viremia was significantly higher in patients during the first 3 months than in those recently infected; however, there were no significant differences in the number of virus-expressing cells per square millimeter of LN tissue in these two groups. Numbers of virus-expressing cells in lymphoid tissue were significantly lower in the subjects with long-term infection than in the other two groups. Therefore, during the transition from primary to chronic HIV infection, the level of HIV replication in lymphoid tissue remains elevated despite the fact that viremia is significantly downregulated. These findings have implications for therapeutic strategies in primary HIV infection and in recent seroconvertors.

Effect of interleukin-2 on the pool of latently infected, resting CD4+ T cells in HIV-1-infected patients receiving highly active anti-retroviral therapy.

The size of the pool of resting CD4+ T cells containing replication-competent HIV in the blood of patients receiving intermittent interleukin (IL)-2 plus highly active anti-retroviral therapy (HAART) was significantly lower than that of patients receiving HAART alone. Virus could not be isolated from the peripheral blood CD4+ T cells in three patients receiving IL-2 plus HAART, despite the fact that large numbers of resting CD4+ T cells were cultured. Lymph node biopsies were done in two of these three patients and virus could not be isolated. These results indicate that the intermittent administration of IL-2 with continuous HAART may lead to a substantial reduction in the pool of resting CD4+ T cells that contain replication-competent HIV.

Effect of tumor necrosis factor alpha on mitogen-activated human B cells.

In this study we demonstrate that the monocyte/macrophage product, tumor necrosis factor alpha (TNF-alpha), has significant in vitro effects of B cell function. It costimulated with anti-mu in the induction of B cell DNA synthesis, and it prolonged the DNA synthesis initiated in B cell cultures stimulated with the human B cell mitogen, Staphylococcus aureus Cowan strain I (SAC). The addition of either IL-1 or IFN-gamma to TNF-alpha resulted in a substantial further increase in DNA synthesis. The addition of TNF-alpha to IL-2, a known inducer of SAC-activated B cell Ig secretion, resulted in a twofold enhancement in the amount of IL-2 stimulated B cell Ig secretion. Receptor binding studies with 125I-TNF-alpha demonstrate a marked increase in TNF-alpha binding sites after B cell activation (approximately 6,000 sites per cell, with an apparent Kd of 2.0 X 10(-10) M). Thus, TNF-alpha may be an important factor in human B cell function and is likely to interact with other T cell and monocyte derived cytokines in the regulation of human B cell proliferation and Ig production.

The efficiency of acute infection of CD4+ T cells is markedly enhanced in the setting of antigen-specific immune activation.

Human immunodeficiency virus (HIV) disease in sub-Saharan Africa generally differs from that observed in the United States and other developed countries in that the risk of seroconversion after exposure is greater and the rate of disease progression to AIDS and death is faster. One theory that could in part explain this difference is the increased state of immune activation associated with a relatively high rate of parasite infestation and other infections among inhabitants of these regions. Using a model based on the cellular microenvironment of lymphoid organs, the role of exposure to HIV during a state of antigen-specific immune activation was investigated. Dendritic cells and CD4+ T cells are the major cellular components of the paracortical region of lymphoid tissue, the primary site of HIV replication. We analyzed cocultures of HIV-pulsed dendritic cells that had matured in the presence of tetanus toxoid and CD4+ T cells before and after inducing an antigen-specific response by in vivo immunization with tetanus toxoid. During antigen-specific immune activation, 100 times less HIV was needed to initiate a productive infection. These findings provide a model system to further delineate the relationship between immune activation and the propagation of HIV infection and suggest a mechanism for the epidemiologic observations of an increased ease of developing HIV infection and faster progression for HIV disease in geographic areas where immune activation is prevalent.

Activation of human B lymphocytes. I. Direct plaque-forming cell assay for the measurement of polyclonal activation and antigenic stimulation of human B lymphocytes.

A model for the detection of single cell antibody production by human tonsillar lymphocytes after stimulation with either sheep red blood cells (SRBC) or polyclonal B-cell activators has been described. The culture system is a modified Mishell-Dutton technique with certain critical factors identified. The assay is a sensitive and resproducible hemolysis-in-gel system employing an ultra-thin layer gel technique measuring plaque-forming cells (PFC) against SRBC targets. Several factors essential for optimal responses are described, but the critical feature of the culture system is the use of selected lots of human AB serum supplements which are extensively absorbed with SRBC. This removes a blocking factor present in most human serum which suppresses the B-cell response to SRBC targets after stimulation with either SRBC or several polyclonal B-cell activators. In addition, absorption of serum with SRBC eliminates the presence of artifactual plaques. Background PFC are extremely low and stimulated cultures show significant and reproducible responses. These studies provide a simple, sensitive, and reproducible model for probing the complex events associated with activation of human B lymphocytes.

Identification, purification, and characterization of antigen-activated and antigen-specific human B lymphocytes.

Activated pneumococcal polysaccharide (PPS)-specific human B lymphocytes have been purified and examined in vitro. They are large cells that are refractory to further activation by antigen, anti-Ig, or Staphylococcus aureus Cowan strain I, but they respond directly by proliferation to B cell growth factors. In addition, they express the isotype pattern of activated cells and a surface marker of activated lymphocytes called 4F2. Finally, they contain a subset of cells that spontaneously secrete PPS-specific antibody.

Factors secreted by human T lymphotropic virus type I (HTLV-I)-infected cells can enhance or inhibit replication of HIV-1 in HTLV-I-uninfected cells: implications for in vivo coinfection with HTLV-I and HIV-1.

It remains controversial whether human T lymphotropic virus type I (HTLV-I) coinfection leads to more rapid progression of human immunodeficiency virus (HIV) disease in dually infected individuals. To investigate whether HTLV-I infection of certain cells can modulate HIV-1 infection of surrounding cells, primary CD4(+) T cells were treated with cell-free supernatants from HTLV-I-infected MT-2 cell cultures. The primary CD4+ T cells became resistant to macrophage (M)-tropic HIV-1 but highly susceptible to T cell (T)-tropic HIV-1. The CC chemokines RANTES (regulated on activation, normal T cell expressed and secreted), macrophage inflammatory protein (MIP)-1alpha, and MIP-1beta in the MT-2 cell supernatants were identified as the major suppressive factors for M-tropic HIV-1 as well as the enhancers of T-tropic HIV-1 infection, whereas soluble Tax protein increased susceptibility to both M- and T-tropic HIV-1. The effect of Tax or CC chemokines on T-tropic HIV-1 was mediated, at least in part, by increasing HIV Env-mediated fusogenicity. Our data suggest that the net effect of HTLV-I coinfection in HIV-infected individuals favors the transition from M- to T-tropic HIV phenotype, which is generally indicative of progressive HIV disease.

Activated B lymphocytes from human immunodeficiency virus-infected individuals induce virus expression in infected T cells and a promonocytic cell line, U1.

Freshly isolated B lymphocytes from patients infected with human immunodeficiency virus (HIV), in contrast to B cells from normal controls, were shown to induce viral expression in two cell lines: ACH-2, a T cell line, and U1, a promonocytic cell line, which are chronically infected with HIV, as well as in autologous T cells. In 10 out of 10 HIV-infected individuals with hypergammaglobulinemia, spontaneous HIV-inductive capacity was found with highly purified peripheral blood B cells, whereas peripheral blood or tonsillar B cells from six healthy, HIV-negative donors did not induce HIV expression unless the cells were stimulated in vitro. The induction of HIV expression was observed in direct coculture experiments of B lymphocytes and HIV-infected cells, and could also be mediated by supernatants from cultures of B cells. Significantly higher amounts of interleukin 6 (IL-6) and tumor necrosis factor alpha (TNF-alpha) were detected in the B cell culture supernatants from HIV-infected patients with hypergammaglobulinemia (IL-6: mean = 536 pg/ml; TNF-alpha: mean = 493 pg/ml), as compared with normal uninfected controls (IL-6: mean = 18 pg/ml; TNF-alpha: mean = 23 pg/ml). Antibodies against these cytokines abolished the HIV-inductive capacity of B cells. We conclude that in vivo activated B cells in HIV-infected individuals can upregulate the expression of virus in infected cells by secreting cytokines such as TNF-alpha and IL-6, and, therefore, may play a role in the progression of HIV infection.

Differential sensitivity of human B cell subsets to activation signals delivered by anti-mu antibody and proliferative signals delivered by a monoclonal B cell growth factor.

The present study demonstrates the minimal, optimal, and synergistic signals involved in the activation of normal human peripheral blood and tonsillar B cells to proliferation. Initial activation signals were delivered to B cells by low concentrations of anti-mu antibody which did not induce proliferation by themselves. However, marked synergy was seen when anti-mu antibody was added to cultures in the presence of monoclonal B cell growth factor (BCGF) obtained from a human T-T cell hybrid such that the B cells underwent substantial proliferation. This latter proliferation was seen without maturation into Ig-secreting cells, which indicates that the BCGF is not a differentiation signal but a signal that drives the cell up to but not beyond the proliferative phase. Of note was the fact that B cells reflected differential sensitivity on the basis of size to either the activation signal delivered by anti-mu antibody or the proliferative signal delivered by BCGF. BCGF directly stimulated the larger B cells in the normal tonsillar B cell repertoire to proliferate without the requirement for an in vitro activation signal, which indicates that the cells had already received some form of activation signal in vivo. Indeed, these cells expressed the 4F2 antigen found on activated but not resting lymphocytes. In contrast, the smaller tonsillar B lymphocytes did not express the 4F2 activation antigen and required activation by anti-mu antibody, which did not of itself induce proliferation, but which acted in synergy with BCGF for substantial proliferation of the B cells. These studies thus provide a useful model of human B cell activation, proliferation, and differentiation and allow a more precise delineation of each phase in this cascade.

Selective abrogation of antigen-specific human B cell responses by antigen-ricin conjugates.

The feasibility of selectively eliminating human antigen-specific B cell responses by treating cells in vitro with antigen covalently linked to a cell toxin was examined. Tetanus toxoid (TT) was conjugated to the toxin ricin via a thioether linkage. Peripheral blood mononuclear cells from recently immunized subjects were preincubated for 2 h with TT-ricin in the presence of lactose. Antigen was then removed, and the cells from recently immunized subjects were preincubated for 2 h with TT-ricin in the presence of lactose. Antigen was then removed, and the cells were stimulated with pokeweed mitogen to induce antibody production. TT-specific antibody production was completely abrogated by preincubation with TT-ricin but not by TT alone or a mixture of TT and ricin. In contrast, polyclonal immunoglobulin production was not diminished by TT-ricin. This selective abrogation was also seen when B cells alone were preincubated with TT-ricin and a source of T cell help was later provided. T cell blastogenic responses to TT remained intact after TT-ricin exposure. Thus, antigen-toxin conjugates are capable of selectively eliminating specific antibody-producing B cell clones, while leaving intact the remainder of the B cell repertoire.

In vitro antigen-induced, antigen-specific antibody production in man. Specific and polyclonal components, kinetics, and cellular requirements.

A highly specific and reproducible antigen-induced, antigen-specific culture and assay system for antibody production by human peripheral blood B lymphocytes has been developed. The system is clearly T cell and monocyte dependent and is independent of exogenous mitogens. The major factors in our ability to trigger specific antibody production with antigen alone have been the use of extremely low concentrations of antigen in vitro (doses several orders of magnitude below those inducing a peak blastogenic response), careful attention to in vitro cell density and culture vessel geometry, and appreciation of the kinetics of the circulating antigen-inducible B cell repertoire. A dichotomy and overlap between antigen-induced, antigen-specific and antigen-induced, polyclonal responses was observed in the study of doubly immunized individuals. Whereas antibody responses highly specific for the antigen in culture were observed under one set of culture conditions (flat-bottomed vessels, 1.5 x 10(6) cells), switching to another culture system (round-bottomed vessels, 5 x 10(5) cells) resulted in polyclonal responses to antigen. Despite these culture condition-related differences in the induction of antibody synthesis, the suppression of specific antibody production that occurred at high concentrations of antigen was specific only for the antigen in culture. The capability to easily and reproducibly look at truly antigen-induced, antigen specific antibody production should be a major tool in furthering the understanding of human B cell activation and immunoregulation.

Development of a human T-T cell hybridoma secreting B cell growth factor.

The success of long-term culture of normal human and murine B cells has been hampered by the limited availability of soluble factors capable of maintaining proliferation of activated B lymphocytes. Previous experiments using various culture-derived supernatants in a human system were unable to separate the activities of B cell growth factor (BCGF) and interleukin 2 (IL-2) by immunochemical means. Thus, purified factors with BCGF activity in the absence of IL-2 activity have not been available for study. In the present study, normal human peripheral blood T cells were fused with the hypoxanthine/aminopterin/thymidine-sensitive human T-leukemic cell line, CEM-6. Supernatants from the resulting hybrid cells were tested for the ability to maintain proliferation of normal human B cells in a recently described assay system for human BCGF. Hybrids demonstrating BCGF activity were cloned by limiting dilution. One hybrid clone, 2B11, continued to support proliferation of B cells in both long-term cultures and 6-d assays at a level significantly above that seen with conventionally produced growth factors. No IL-2 activity was found in the supernatant from hybrid 2B11. The hybridoma supernatant was fractionated by gel filtration, and maximum proliferation of B cells was supported by the 18-20,000 mol wt protein fraction. Thus, a human T-T cell hybridoma that has BCGF activity in the absence of any demonstrable IL-2 activity has been developed. Human T-T cell hybridomas secreting discrete immunoregulatory factors should prove to be powerful tools in dissecting the mechanisms of immunoregulation of human lymphocyte function.

Chédiak-Higashi gene in humans I. Impairment of natural-killer function.

Natural-killer (NK)-cell function was profoundly depressed in donors homozygous for the Chediak-Steinbrinck-Higashi syndrome (C-HS) gene when compared with age- and sex-matched normals. This apparent defect was not simply a result of a delayed response because little cytolysis was evident in kinetics experiments even after 24 h of incubation. NK cells from C-HS donors failed to lyse adherent (MDA, CEM, and Alab) or nonadherent (K562 and Molt-4) tumor cell lines or nontransformed human fetal fibroblasts. Therefore, the apparent C-HS defect was not a result of a shift in target selectivities. In addition, the depressed reactivity did not appear to be a result of suppressor cells or factors because: (a) enriched NK populations (nonadherent lymphocytes bearing receptors for the Fc portion of IgG) from C-HS donors were low in NK-cell function, (b) C-HS mononuclear cells did not inhibit the cytotoxicity of normal cells in coculture experiments, and (c) cells from the C-HS donors remained poorly reactive even after culture for up to 7 d. The nature of the defective NK activity in C-HS patients is not clear but may lie within the lytic mechanism rather than at the level of the recognition structure or population size because the frequency of target-binding cells was normal. In vitro NK activity could be partially restored by interferon treatment. Combined with the results presented in the following paper (4), these observations suggest that the C-HS gene causes a selective immunodeficiency disorder, mainly involving NK cells. This finding, in conjunction with the high incidence of spontaneous possibly malignant, lymphoproliferative disorders in these patients, may have important implications regarding the theory of immune surveillance mediated by NK cells.

The Bb fragment of complement factor B acts as a B cell growth factor.

The process of B cell growth and differentiation into plasma cells is highly regulated and may be influenced by a large number of inflammatory mediators, including complement components. We have studied the regulatory influence of Bb, a 60-kD peptide created during the cleavage of complement Factor B by Factor D and C3b. Purified Bb alone had no effect on proliferation and differentiation of human splenic or tonsillar B cells. However, when B cells were activated by Staphylococcus aureus Cowan I (SAC), Bb enhanced proliferation in a dose-dependent manner. Bb also enhanced proliferation when cocultured with SAC and suboptimal concentrations of purified 60-kD B cell growth factor (HMW-BCGF), a previously described lymphokine that is known to possess growth-promoting activity. However, Bb had no effect on cells treated with optimal concentrations of HMW-BCGF. Like HMW-BCGF, Bb's major effect was on the larger in vivo activated B cells. Half-maximal enhancement of proliferation was reached at a Bb concentration of 1-10 nM. Of note is the fact that antibody to Factor B recognized HMW-BCGF, and an mAb to HMW-BCGF also recognized Factor B and Bb, but not Ba. Moreover, radiolabeled Bb bound saturably to activated B cells and to an EBV-transformed human B cell line. The binding of Bb was inhibited by HMW-BCGF but not by Ba or IgG. Thus, Bb is antigenically and functionally related to HMW-BCGF, and can act as a B cell growth and differentiation factor at potentially physiologic concentrations. These data suggest that Bb may be important in amplifying the immune response in areas of inflammation. Since complement activation occurs at inflammatory sites long before induction of HMW-BCGF synthesis, Bb may be an early signal for the clonal expansion of antigen-activated B cells.

Purification to homogeneity of a high molecular weight human B cell growth factor; demonstration of specific binding to activated B cells; and development of a monoclonal antibody to the factor.

High molecular weight B cell growth factor (HMW-BCGF) produced by a T cell line was purified to homogeneity and demonstrated to bind specifically to activated human B cells. A monoclonal antibody to HMW-BCGF was developed that (a) specifically inhibited the activity of HMW-BCGF in enhancing B cell proliferation, (b) specifically bound to HMW-BCGF in Western blots, (c) specifically absorbed HMW-BCGF activity from culture supernatants, and (d) specifically absorbed an internally labeled protein from T-ALL supernatant which comigrates with HMW-BCGF on sodium dodecyl sulfate-polyacrylamide gels. This antibody should help in cloning the gene for HMW-BCGF and further exploring the physiologic roles of HMW-BCGF.

cDNA cloning of the B cell membrane protein CD22: a mediator of B-B cell interactions.

We have cloned a full-length cDNA for the B cell membrane protein CD22, which is referred to as B lymphocyte cell adhesion molecule (BL-CAM). Using subtractive hybridization techniques, several B lymphocyte-specific cDNAs were isolated. Northern blot analysis with one of the clones, clone 66, revealed expression in normal activated B cells and a variety of B cell lines, but not in normal activated T cells, T cell lines, Hela cells, or several tissues, including brain and placenta. One major transcript of approximately 3.3 kb was found in B cells although several smaller transcripts were also present in low amounts (approximately 2.6, 2.3, and 1.6 kb). Sequence analysis of a full-length cDNA clone revealed an open reading frame of 2,541 bases coding for a predicted protein of 847 amino acids with a molecular mass of 95 kD. The BL-CAM cDNA is nearly identical to a recently isolated cDNA clone for CD22, with the exception of an additional 531 bases in the coding region of BL-CAM. BL-CAM has a predicted transmembrane spanning region and a 140-amino acid intracytoplasmic domain. Search of the National Biological Research Foundation protein database revealed that this protein is a member of the immunoglobulin super family and that it had significant homology with three homotypic cell adhesion proteins: carcinoembryonic antigen (29% identity over 460 amino acids), myelin-associated glycoprotein (27% identity over 425 amino acids), and neural cell adhesion molecule (21.5% over 274 amino acids). Northern blot analysis revealed low-level BL-CAM mRNA expression in unactivated tonsillar B cells, which was rapidly increased after B cell activation with Staphylococcus aureus Cowan strain 1 and phorbol myristate acetate, but not by various cytokines, including interleukin 4 (IL-4), IL-6, and gamma interferon. In situ hybridization with an antisense BL-CAM RNA probe revealed expression in B cell-rich areas in tonsil and lymph node, although the most striking hybridization was in the germinal centers. COS cells transfected with a BL-CAM expression vector were immunofluorescently stained positively with two different CD22 antibodies, each of which recognizes a different epitope. Additionally, both normal tonsil B cells and a B cell line were found to adhere to COS transfected with BL-CAM in the sense but not the antisense direction.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of HIV-1 replication in latently infected CD4+ T cells using a combination of cytokines.

Although it has been demonstrated that certain cytokines, particularly proinflammatory cytokines, can enhance ongoing viral replication in peripheral blood mononuclear cells (PBMCs) of HIV-1-infected individuals, it is unclear what role these cytokines play in the induction of HIV-1 replication in latently infected, resting CD4(+) T cells. This study demonstrates that the in vitro combination of the proinflammatory cytokines interleukin (IL)-6 and tumor necrosis factor (TNF)-alpha together with the immunoregulatory cytokine IL-2 are potent inducers of viral replication in highly purified, latently infected, resting CD4+ T cells derived from HIV-infected individuals who are antiretroviral therapy-naive as well as those who are receiving highly active antiretroviral therapy (HAART). Viral replication induced by this combination of cytokines was completely suppressed in the presence of HAART in vitro. Given that an array of cytokines, including IL-6, TNF-alpha, and IL-2, are copiously expressed in the microenvironment of the lymphoid tissues, which harbor the latent viral reservoirs, induction of HIV by this combination of cytokines may in part explain the commonly observed reappearance of detectable plasma viremia in HIV-infected individuals in whom HAART was discontinued. Moreover, since it is likely that these infected cells die upon activation of virus and that HAART prevents spread of virus to adjacent cells, the observation that this combination of cytokines can markedly induce viral replication in this reservoir may have important implications for the activation-mediated diminution of the latent reservoir of HIV in patients receiving HAART.

Selective suppression of an early step in human B cell activation by cyclosporin A.

The effect of cyclosporin A (CsA), a fungal metabolite with immunosuppressive properties, on the induction of human B cell proliferation and differentiation, has been described. CsA had a selective inhibitory effect on the activation phase of the cell cycle vs. the proliferation phase following preactivation of the cells. Cell enlargement and RNA synthesis of small resting B cells triggered by anti-mu were inhibited by addition of CsA (5-500 ng/ml). The inhibitory effect of CsA was found only when the drug was added within 24 h of initiation of culture. In marked contrast, once small B cells were activated by anti-mu, the resulting large, activated B cells could be induced to initiate DNA synthesis by incubation with B cell growth factor (BCGF), and addition of CsA (1-1,000 ng/ml) to the culture did not suppress this BCGF-induced B cell proliferation. Addition of CsA to cultures of B cells which had been preactivated with Staphylococcus aureus Cowan strain I (SAC) and were already proliferating did not suppress B cell differentiation factor (BCDF)-induced differentiation of these cells. Thus, these data indicate that CsA can be used as a pharmacologic tool to dissect out human B cell responses into two distinct steps: (a) the initial activation step induced by anti-Ig, which is characterized by cell enlargement, RNA synthesis, and expression of receptors for BCGF; and (b) the proliferative step induced by BCGF in these preactivated B cells that undergo DNA synthesis and can then go on to differentiate in the presence of BCDF. In this regard, CsA selectively suppresses an early step of human B cell activation and has little inhibitory effect on the subsequent factor-dependent proliferation and differentiation.