Development Of Plaque-forming Cells Research Articles

Direct interaction between an antigen-specific B cell clone and an MHC class II-reactive helper T cell clone.

TP67.14 established by somatic hybridization is a 2,4,6-trinitrobenzenesulfonic acid (trinitrophenyl, TNP)-specific B cell clone with a receptor molecule for TNP on the cell membrane, and MS202 is an interleukin-2 (IL-2)-dependent T helper (Th) cell clone reactive to auto-MHC class II antigens (IAk and IEk) as previously reported. In the present study it was shown that MS202 considerably induced the maturation of TP67.14 into anti-TNP plaque-forming cells (PFCs), and this response was markedly augmented by the addition of TNP-keyhole limpet hemocyanin (KLH). Recombinant cytokines and the culture supernatant of MS202 with TP67.14 did not affect the generation of anti-TNP antibodies by TP67.14. Also, neither anti-IL-4 nor anti-IL-5 monoclonal antibody (mAb) inhibited the maturation of TP67.14 mediated by MS202. The differentiative effect of MS202 on TP67.14 was completely lost when each cell was separately cultured using a semipermeable membrane. Monoclonal antibodies against LFA-1 beta molecules significantly blocked the development of anti-TNP PFCs induced by MS202, as well as anti-IAk and anti-IEk mAbs. Interestingly, the plasma membrane-enriched fraction (PM) derived from MS202 exhibited much more differentiative effects on TP67.14 treated with TNP-KLH than PM from other T cell lines and concanavalin A-induced T lymphoblasts. In addition, TNP-conjugated PM from MS202 by itself induced a great number of anti-TNP PFCs. The present findings indicate that MS202 is capable of inducing the maturation of TP67.14, which is considered to represent a population of B cells with antigen specificity in a late lineage of B cell maturation, through direct cell contact but not soluble factors. This suggests that B cells with antigen specificity, in the presence of antigen, can be induced to mature into antibody-secreting cells through direct contact with Th cells; in this process surface major histocompatibility complex class II and lymphocyte function-associated antigen 1 (LFA-1) molecules are directly involved and the cell membrane derived from Th cells provides a transductional signal for maturation of B cells with antigen specificity in the presence of antigen.

Two populations of guinea pig erythrocyte-rosetting cells in the cat: Evidence for their T-helper function in mitogen-induced synthesis of Ig and interleukin-2

Studies in our laboratory have shown that T-helper (T-H) and T-suppressor (T-S) cells in cat peripheral blood leukocytes (PBL) rosette with guinea pig (GP) and gerbil (G) erythrocytes (E), respectively. Removal of GE-rosetting cells leads to an enhanced (two- to threefold) synthesis of Ig in a pokeweed mitogen (PWM)-driven system as measured by plaque-forming cells (PFC) to protein A-coated sheep RBC, while depletion of GPE-rosetting cells yields a PFC response only 10–15% of the control. Surprisingly, removal of both GE- and GPE-rosetting cells gave a response equivalent to 40–100% of the control PBL. Analysis of the mixed GE/GPE rosette depleted cultures revealed the reappearance of GPE- but not GE-rosetting cells, reaching maximum values within 12–18 hr after in vitro culture. Cultures of control PBL and those following the mixed rosette depletion showed two populations of GPE-rosetting cells; the GPE-1 cells, present on Day 0 before culture, and the GPE-2 cells, those appearing on Day 1. Addition of cycloheximide prevented development of the GPE receptor while colchicine and mitomycin C were without effect. The development of PFC after the mixed GE/GPE rosette depletion was interpreted as being due to the GPE-2 cells functioning as T-H cells in the absence of any T-S (GE-rosetting) cells. This thesis was supported by showing a marked decrease in the PWM-induced Ig response when both the GPE-1 and GPE-2 populations were removed on Day 1. Additional evidence for functional T-H cells in the GPE-rosetting population was obtained by analyzing interleukin-2 (IL-2) production. Removal of the GPE-rosetting cells (GPE-1 and/or GPE-2) from PBL led to a marked decrease in Con A-induced IL-2 synthesis while removal of the GE-rosetting cells yielded a normal or slightly greater than normal response.

Activation of Murine Autoreactive B Cells by Interleukin 1-Like Factors Released from Synovial Inflammatory Cells of Rheumatoid Arthritis Patients

The effect of interleukin 1 (IL-1)-like factor(s), produced by cells isolated from the synovial fluids of rheumatoid arthritis (RA) patients, on an in vitro murine model of spontaneous autoimmunity, i.e., the development of plaque-forming cells (PFC) to bromelain-treated mouse red blood cells (Br-MRBC) in mouse peritoneal cell (PC) cultures, has been investigated. It has been found that IL-1-containing culture supernatants from cells isolated from joint fluids of RA patients, as well as recombinant IL-1, determine a marked increase in anti-Br-MRBC PFC development. Moreover, factor(s) of 10-20 KD molecular weight, with IL-1-like biological activity, capable of increasing the anti-Br-MRBC PFC development in mouse PC cultures, have been demonstrated in joint fluids from RA patients. The finding that synovial inflammatory cells produce factors that activate autoreactive B cells further supports the role of autoimmunity in the pathogenesis of rheumatoid arthritis, as self-perpetuing disorder.

Enhancement of anti-sheep erythrocyte plaque-forming cell levels by cimetidine in vivo

Previous studies demonstrated an immunomodulatory role for cimetidine in contact sensitivity responses in vivo. The present experiments have examined the effects of cimetidine upon in vivo antibody production. When given intravenously, cimetidine treatment consistently resulted in enhanced numbers of IgM plaque-forming cells (PFC) following immunization with sheep erythrocytes. This enhancement was dependent upon both the dose and timing of cimetidine administration. In timing experiments, maximal enhancement of PFC levels was obtained when cimetidine was given at the time of or within three days following immunization. Treatment with cimetidine four days after immunization or one day before assay led to a slight decline in PFC level. In contrast, intraperitoneal treatment with cimetidine has little effect upon the anti-sheep erythrocyte PFC level regardless of dose or timing. Overall, these results indicate that cimetidine may be capable of modulating PFC development in vivo. This modulation, however, depends critically upon dose, timing, and route of cimetidine administration.

The effect of maternal antigenic stimulation upon the active immune responsiveness of their offspring: suppression induced by soluble protein antigen, ovalbumin, in mice.

The active immune responsiveness of the offspring of pregnant mice stimulated with heterologous protein antigen was investigated by measuring the plaque-forming cells (PFC). Mice (C57BL/10;B10) immunized once in pregnancy with ovalbumin (OVA) in the form of Al(OH)3 gel (in alum) or in a soluble form (in saline) developed no anti-OVA PFC response. The anti-OVA PFC response suppression induced in the offspring was high in the offspring of alum-treated mothers and low in those of saline-treated mothers. The optimal dose of OVA in alum that induces the highest immunological memory in pregnant mice caused the complete suppression of PFC development in their offspring. The same dose of OVA in saline induced a negative immunological memory in pregnant mice and partial suppression in the offspring. On the other hand, mice primed prior to conception and boosted during pregnancy developed anti-OVA PFC in significant numbers, and only a partial suppression was established in their young. Based on these data, we discussed the possible mechanisms concerned with the specific suppression induced in the young B10 mice stimulated by OVA.

Functional Analysis of Pokeweed Mitogen‐Dependent Cell Interactions in Murine Spleen Cells

The nature of lymphocyte responses on addition of pokeweed mitogen (PWM) to normal murine spleen cells was studied in low cell density cultures. PWM, over a wide range of concentrations, stimulated proliferation in a set of cells roughly 10-fold smaller than the lymphocyte populations responding to either concanavalin A or lipopolysaccharide. PWM also induced a relatively small number of B lymphocytes in these cultures to mature to Ig-secreting plaque-forming cells (PFC). Proliferative and PFC responses were completely abrogated by T-cell removal from normal spleen cell cultures. Moreover, cell mixture and irradiation experiments demonstrated that B lymphocytes do not proliferate in response to PWM, even in the presence of an excess of normal T cells, suggesting that PFC development results from terminal maturation without proliferation. Finally, parallel titrations of cloned helper cells, normal splenic T cells or T-cell blasts induced by PWM showed that the poor B-lymphocyte responses in normal spleen cell cultures is due to the very low frequency of competent helper cells in these populations. PWM, however, was competent to activate and expand this set of helper lymphocytes in primary cultures.

Effect of Maternal Antigenic Stimulation on the Active Immune Responses of Their Offspring

Pregnant mice were stimulated by heterologous erythrocyte and protein antigen, and the active immune responses of their offspring as measured by plaque-forming cells (PFC) were investigated. In offspring derived from mothers immunized by a suitable amount of T-dependent antigen, clear-cut suppression of development of specific PFC in spleen was observed over a significant period after delivery. The mechanism of this suppression was investigated, and the following results were obtained. When the heterologous erythrocyte was used as antigen, the more the specific PFC developed in the mother spleen, the stronger the suppression of PFC observed in their offspring. However, it is worthy of note that passive administration of antibody to pregnant mice did not induce suppression in their young. In case of the protein antigen ovalbumin (OVA), pregnant mice had to be injected with a suitable amount of antigen, along with aluminium hydroxide, for either primary or secondary stimulus to induce the suppression of specific PFC in their offspring. Soluble OVA administered to pregnant mice was not effective for inducing suppression in the offspring. Based on these results, some possible mechanisms are discussed concerning specific PFC suppression in the offspring when pregnant mice are stimulated.

Clustering of dendritic cells, helper T lymphocytes, and histocompatible B cells during primary antibody responses in vitro.

Mouse spleen suspensions generate discrete cell clusters within 1-2 d of culture. We have isolated these clusters by velocity sedimentation to study their contribution to primary antibody responses. Clusters represent approximately 5% of the starting spleen cells and consist of 20-50% B cells, 20-50% T cells, and 10-20% dendritic cells (DC). When the cultures are stimulated with thymus-dependent antigens, like heterologous red cells or dinitrophenyl-keyhole limpet hemocyanin (DNP-KLH), the clusters are the principal site for the development of plaque-forming cells (PFC). Noncluster fractions form few PFC and only when supplemented with fresh DC. PFC responses in all cases are antigen specific. B cells cluster only in the presence of T cells and DC (1 DC/200 B-T cell mixtures) and only after encountering specific antigen. The elimination of either DC or Lyt-1+2- T cells, with monoclonal antibody and complement, ablates B cell development into PFC. PFC responses are restored with antigen-nonspecific helper factors formed in the syngeneic mixed leukocyte reaction between DC and T cells. Since PFC to DNP-KLH do not develop de novo when B cells are exposed to antigen and helper factors, anti-DNP PFC precursors must be stimulated within clusters to become responsive to helper factors. PFC development within clusters is restricted by the major histocompatibility complex (MHC). When DC and T cells are from strain P1, then P1 but not P2 B cells develop into PFC; when DC are from strain P2 and T cells from strain P1, strain P2 B cells are selected to become PFC in clusters. The entry of B cells into clusters is itself MHC restricted, since P1 DC/T cells aggregate six times as many B cells from strain P1 as strain P2. Thus, clusters are the site in which DC, B, and T cells interact to generate PFC. One can use clusters to retrieve B cells that have been selected in an antigen-dependent, MHC-restricted fashion and to show that clustering B cells become responsive to soluble, polyclonal helper factors.

T cells and the anti-trinitrophenyl antibody response to fetal calf serum and 2-mercaptoethanol.

Unprimed murine lymphocytes maintained in culture medium containing fetal calf serum (FCS) and 2-mercaptoethanol (2-ME) developed very high levels of anti-trinitrophenyl (TNP) plaque forming cells (PFC). Both FCS and 2-ME contributed to the response. The development of anti-TNP PFC during culture was accompanied by a 10-fold expansion in the number of immunoglobulin-secreting cells, indicating polyclonal stimulation. However, the number of anti-TNP PFC was disproportionately high and not accompanied by a similar increase in plaques specific for sheep red blood cells. The TNP-specific plaques were not artifacts of the plaque assay since they were 98% inhibited by specific antigen. The in vitro induction of anti-TNP PFC by FCS and 2-ME was common to a number of mouse strains, although some genetic variation occurred. Nylon-wool-separated B cells, nude mouse spleen cells, and bone marrow cells all produced high levels of anti-TNP after culture with medium containing FCS and 2-ME. The addition of T cells to B-cell cultures increased the numbers of anti-TNP PFC by 1.5- to 2.5-fold. The presence of a TNP-cross-reacting antigen in FCS probably contributed to the unexpectedly high anti-TNP response. The response to the antigen in FCS was potentiated by the enhancing activity of 2-ME.

IgG subclass distribution of anti-sheep red blood cell plaque-forming cells in mice with the CBA/N defect.

The IgG subclass distribution of anti-sheep red blood cell (SRBC) plaque-forming cells (PFC) in CBA/N, CBA/Ca, and (CBA/N X CBA/Ca)F1 male and female mice was examined. The results show that the X-linked immunodeficiency of CBA/N mice affects the development of anti-SRBC PFC of all IgG subclasses tested.

LPS-induced enhancement of plaque-forming cell response to bromelain-treated syngeneic erythrocytes in mouse peritoneal cell cultures.

The effect of bacterial lipopolysaccharide (LPS) on the development of plaque-forming cells (PFC) against bromelain-treated syngeneic mouse red blood cells (Br-MRBC) was studied in peritoneal cell (PC) cultures. It was found that LPS enhances the development of PFC to Br-MRBC and increases DNA synthesis in PC cultures. The LPS-induced enhancement of PFC to Br-MRBC, however, does not appear to require cell proliferation, since it also occurred in PC cultures pretreated with mitomycin C. In addition, the LPS-induced B lymphocytes blastogenesis is under the control of macrophages, while cell differentiation of precursor B lymphocytes into cells actively producing antibodies against Br-MRBC is regulated by suppressor T lymphocytes.

Effects of Corynebacterium parvum on depressed immunological reactions in EL-4-tumor-bearing mice

Effects of Corynebacterium parvum on the development of plaque-forming cells (PFC), cell-mediated cytotoxicity (CMC), and delayed footpad reaction (DFR) to chicken erythrocytes (CRBC) were investigated in EL-4-bearing syngeneic mice. PFC, CMC, and DFR responses after the primary immunization were suppressed in tumor-bearing mice and restored by C. parvum treatment. PFC and CMC responses in tumor-bearing mice were restored by the transfer of spleen cells of C. parvum-treated normal mice. Such powers of recovery were abrogated by the removal of glass-adherent cells but not by the removal of θ-positive or Ig-positive cells. DFR was suppressed not only in the primary but also in the secondary immunization, in contrast to PFC and CMC; the secondary responses of these types were not suppressed in tumor-bearing mice. Positive DFR was not elicited in tumor-bearing mice after adoptive transfer of sensitized lymphocytes from normal immune donors. The DFR became positive in such tumor-bearing recipients when they were treated by C. parvum. Macrophage functions in the induction phase of the immune response as accessory cells and in the expression of DFR as secondary cells appear to be suppressed in tumor-bearing mice and restored by C. parvum.

Heavy metal modulation of lymphocyte activities: I. In vitro effects of heavy metals on primary humoral immune responses

The ability of heavy metals to modulate in vitro primary humoral immune responses has been investigated. The relative immunosuppresive activities of the heavy metals tested were Hg 2+ > Cu 2+ > Mn 2+ > Co 2+; Cd 2+ > Cr 3+; Sn 2+; Zn 2+. Fe 2+ had no significant effect on the development of SRBC-specific plaque-forming cells (PFC), and Pb 2+ and Ni 2+ enhanced the PFC response. The immunosuppressive activities of the heavy metals usually correlated with their toxicity and their inhibition of lymphocyte proliferation. The immunopotentiating metals, Pb 2+ and Ni 2+, alone induced lymphocyte proliferation, and they increased the proliferative response induced by LPS and 2-ME but not Con A and PHA. Although Pb 2+ and Ni 2+ were the only two heavy metals tested which enhanced lymphocyte reactivity, they did not appear to function via similar mechanisms. Pb 2+ appeared to enhance the development of PFC by directly interacting with lymphocytes and altering their activity since lymphocyte preincubation with Pb 2+ was sufficient for enhancement; whereas, lymphocyte preincubations with Ni 2+ did not induce enhancement. Pb 2+ did not appear to alter the immunogenicity of the antigens employed; however, pretreatment of SRBC with Ni 2+ eliminated their ability to stimulate a SRBC-specific response in vitro. Inhibition of the PFC response by the chelator EGTA could be reversed by Ca 2+ or Ni 2+, but not by Cu 2+, Hg 2+, Pb 2+, or Zn 2+. The stimulatory activity of Pb 2+ and Ni 2+ could not be accounted for by significant alteration of the antigenicity of the lymphocytes, because heavy metal preincubated syngeneic cells were not stimulatory for untreated syngeneic cells. On the other hand, Pb 2+ and Ni 2+ did slightly enhance the mixed lymphocyte culture (MLC) response to allogeneic cells; Hg 2+ inhibited the MLC response.

Immunoglobulin C-gene expression. I. The commitment to IgG subclass of secretory cells is determined by the quality of the nonspecific stimuli.

Polyclonal stimulation of normal splenic B lymphocytes with either lipopolysaccharide (LPS) or helper T lymphocytes specific for B cell surface antigens results in the selective expression of IgG subclasses by the secretory cells: in addition to IgM-secreting plaque-forming cells (PFC), thymus-independent stimulation leads to the development of IgG2 and IgG3 PFC, while helper cell-dependent activation leads to IgG1 and IgG2 PFC. This cannot be solely explained by selective stimulation of distinct B cell subpopulations, because purified LPS-reactive blasts if restimulated by helper cells switch to IgG1 while if maintained with LPS switch to IgG3. The simultaneous stimulation of splenic B cells with LPS and helper cells results in additive IgM and IgG2 responses, but in the selective suppression of IgG3 PFC with a concomitant synergic enhancement of IgG1 responses. These results are interpreted to indicate that the expression of IgG C genes in proliferating B lymphocytes is directed by the quality of nonspecific stimuli.

The lipoprotein of the outer membrane of Escherichia coli: a B-lymphocyte mitogen.

The lipoprotein of the outer membrane of Escherichia coli is a B-cell mitogen in mice. Polyclonal activation of B lymphocytes was measured by an increase in thymidine uptake, by the development of plaque-forming cells against densely coupled trinitrophenylated sheep red cells, and by selectively increased rates of synthesis and secretion of leucine-labeled IgM. Murein-free and muropeptides-containing lipoprotein are effective in B-cell activation, while free murein is inactive. Removal of ester-linked fatty acids from the amino-terminal end of the lipoprotein by alkaline hydrolysis abolishes the mitogenicity of the lipoprotein. B lymphocytes from high responder (C3H/Tif and BALB/c nu/nu) or from low responder (C3H/HeJ) mice to the mitogen lipopolysaccharide (LPS) both respond well to the lipoprotein. Anti-immunoglobulin antibodies inhibit the mitogenic stimulation of B cells by lipoprotein. A complex of structures including the Ig-receptor molecules, the LPS receptor, and the lipoprotein receptor appear involved in the regulation of mitogenic stimulation of B cells to proliferation and differentiation to IgM-secreting cells.

The Expression on Mouse Lymphoid Cells of Th-B, an Antigen Common to Mouse B Cells and Thymus Cells

Abstract A new cell-surface antigen of mouse lymphocytes, which was found earlier on normal and malignant plasma cells and about half the normal thymus cell population, has been shown to be on mature B cells and on the least immunocompetent subpopulation of cells of the thymus. The antigen was first detected by the cytotoxic effect on relevant cells of an in vivo purified rabbit antiserum raised against cells of the mouse IgM-producing plasma cell tumor MOPC-104E. We have now used a cell-transfer method to examine the effect of this antiserum (RantiM104E) on immunocompetent lymphocytes of spleen, including separated splenic B and T cells and thymus and bone marrow cells. We have found the antigen on the immunocompetent B lymphocytes of the spleen, but not on any other of the immunocompetent cells examined. The method involves determining the effect of the antiserum on the development of plaque-forming cells in the spleens of irradiated mice that have received lymphocytes from immune or non-immune donors by treating the lymphocytes with RantiM104E antiserum before transfer. Plaque-forming cells are suppressed when antiserum-treated spleen cells are transferred to the irradiated mice. This suppression was found to be due to action on only the splenic B cells. The helper activity of splenic T cells was not reduced significantly by antiserum treatment. Neither was the helper activity of thymus cells similarly treated reduced by the antiserum. We found, in fact, that the subpopulation of the thymus cells which is resistant to the antiserum was more immunocompetent than the total thymus cell population. In addition, the hydrocortisone-resistant thymus cells were also found to be resistant to the cytotoxic action of the antiserum. The antiserum had no effect on bone marrow cells in the cell-transfer procedure. The antigen involved, which we are designating “Th-B” appears to be a B cell line marker which appears relatively early during the differentiation of mouse B cells from precursor cells and is lost during maturation of T cells.

The kinectics of proliferation and maturation of mitogen-activated bone marrow-derived lymphocytes.

AbstractThe B cell mitogens lipopolysaccharide (LPS), purified protein derivative of tuberculin (PPD) and fetal calf serum (FCS) all stimulate spleen cells from nude mice to increased DNA‐synthesis 14 to 16 h after the initiation of mitogenic stimulation. The three mitogens also induce the maturation of B cells to plaque‐forming cells (PFC). All three mitogens stimulate largely identical B cell populations. The extent to which B cells are stimulated to DNA synthesis and to the development of PFC varies from mitogen to mitogen.PFC arise by clonal growth through proliferation and maturation. A “hot pulse” of radioactive thymidine given between 0 and 20 h of stimulation has no effect on the development of PFC during subsequent mitogenic stimulation. Pulses given between 24 and 36 h after stimulation abolish the development of over 90 % of the PFC. The minimum pulse time required for maximum inhibition of the development of PFC during that time is 6 h. Beyond 36 h of stimulation by either of the three mitogens, PFC emerge rapidly. Between 60 and 72 h less than 10 % of all PFC emerging at 72 h are abolished by a “hot pulse”.The kinetics of the clonal development of PFC after mitogenic stimulation are compared to that of antigenic stimulation. The comparison suggests that B cells stimulated by antigen in the in vivo primary immune response culture system go through a longer period of proliferation than when they are stimulated by mitogen before maturation to PFC.

Regulation of the Antibody Response to Type III Pneumococcal Polysaccharide

Abstract The present study was conducted primarily to obtain information concerning the mechanism by which suppressor thymic-derived (T) cells influence the magnitude of the antibody response to type III pneumococcal polysaccharide (SSS-III). This was accomplished by examining the effects of treatment with antilymphocyte serum (ALS) and Velban, a mitotic inhibitor, on the kinetics for the appearance of plaque-forming cells (PFC) in mice immunized with SSS-III. Without ALS-treatment, the kinetics were essentially biphasic. First, there was a phase during which PFC appeared at a relatively rapid rate; this was followed by a phase during which the rate of appearance of PFC gradually declined until maximal numbers were attained. In contrast, similar rate changes were not noted for mice given ALS; here, PFC increased at a uniform exponential rate throughout the 1st 5 days of the antibody response. Velban completely arrested the development of additional PFC in ALS-treated but not non-ALS-treated mice. These findings indicate that suppressor T cells act primarily by limiting the extent to which antibody-forming bone marrow-derived cells proliferate following immunization.

Detergent effects on the development of antibody-producing cells in vitro: a model for the study of chemical effects on cell differentiation in a developing system.

Mishell-Dutton’s spleen cell model of immunity development was used to study the effects of common detergents on production of plaque forming cells (PFC). At 100 ppm the anionic detergent Tide XK and sodium dodecyl sulfate (SDS) inhibit PFC development by approximately 50%. Nitrilotriacetic acid (NTA) and Haemo-Sol are non-inhibitive. Nonionic Triton X-100 completely inhibited PFC production at 10 ppm. Cationic cetyltrimethylammonium bromide (CTM) added on explantation day gives 80% inhibition at 0.01 ppm. Cytotoxicity of CTM appeared at lower concentrations than PFC inhibition. Though less favorable to undesired algal growth, cationic and nonionic detergents inhibit antibody development in vitro at low concentrations and may have a teratogenic potential in vivo.

Antibody Production by Host Cells in X-Irradiated Recipients of Allogeneic Spleen Cells

Abstract Experiments were performed to test the hypothesis that two different cells, a radiosensitive antigen reactive cell and a radioresistant precursor of the antibody producing cell collaborate in antibody synthesis. Twenty-four hours following 500 R x-irradiation, experimental recipient group C mice were injected with normal allogeneic spleen cells and sheep red blood cells (SRBC). Control recipients of group A were irradiated similarly and injected with SRBC alone while control recipients of group B were irradiated similarly and injected simultaneously with SRBC and allogeneic spleen cells from heavily irradiated (1000 R) mice. Five to 10 days after antigen injection, the numbers of plaque-forming cells (PFC) of donor and host origin were determined in the spleens of the three groups. The contribution of donor and host PFC was distinguished by the use of anti-histocompatibility sera. On day 5, the spleens of mice in group C contained 26 times as many PFC as did the spleens of mice in group A. Of these, 80% were donor cells and 20% were of host type. Thus more than five times as many host PFC developed in group C than in group A. At least some of the stimulatory effect on host PFC was independent of the normal donor spleen cells because the spleens of control group B contained twice as many PFC as did those of control group A. On days 6 and 7, the PFC in group C decreased, while those in control group A increased, possibly due to alloimmune reactions between donor graft and recipient. The injection of antidonor H-2 serum into group C on day 3 suppressed the development of PFC of both donor and recipient type. Evidently survival of donor cells beyond day 3 was necessary for the enhanced recipient PFC response. It is concluded that existing data establish the importance of cell interactions in the immune response but do not prove the existence of an antigen-sensitive cell capable of transmitting antigen-specific information to the precursor of the antibody-producing cell.