Strain dependency of the immunopotentiating activity of Lactobacillus delbrueckii subsp. bulgaricus

To obtain strains of Lactobacillus delbrueckii subsp. bulgaricus with high immunopotentiating activity, we screened 90 strains of this bacterial species for the proliferative response of murine spleen and beta-lactoglobulin-primed lymph node cells. In this screening, certain strains showed strong immunopotentiating activity. Among them, strain 1023 had the strongest mitogenic activity for murine Peyer's patch (PP) cells. Furthermore, strain 1023 induced IgA antibody production by PP cells as strongly as Bifidobacterium longum 6001, which had adjuvant activity when orally administered. Also in the assays using immune cells from human-flora-associated mice a few strains including 1023 showed strong immunopotentiating activity comparable to B. longum 6001. These results suggest that L. delbrueckii subsp. bulgaricus strains such as 1023 may be useful for the production of fermented milk with a more beneficial effect on the systemic and mucosal immune system.     

Protective effect of biological response modifiers on murine cytomegalovirus infection

Pretreatment with two biological response modifiers (BRM), OK-432 and PS-K, protected mice from lethal infection by murine cytomegalovirus (MCMV). This was evidenced by an increase in 50% lethal doses and a decrease in titers of infectious viruses replicated in the liver and spleen. Spleen cells from the BRM-treated mice augmented the natural killer (NK) cell activity and suppressed the replication of MCMV in vitro. During MCMV infection, the NK cell activity of the spleen cells was maintained at a high level in the BRM-treated mice, whereas it was severely impaired in untreated mice. The BRM-induced protection was nullified by concomitant administration of antiasialo GM1 antibody. Interferon was neither induced by BRM treatment nor enhanced in BRM-pretreated and MCMV-infected mice. Thus, the protective effect of OK-432 and PS-K seems to be based on activation of NK cells and prevention of MCMV-induced inhibition of the NK cell activity.     

Immunopotentiating activity of nigerooligosaccharides for the T helper 1-like immune response in mice

The immunopotentiating activity of nigerooligosaccharides (NOS), a mixture of nigerose, nigerosyl glucose and nigerosyl maltose, was studied in vitro and in vivo in mice. Mitogen-induced proliferation of splenocytes from normal mice was augmented in a dose-dependent manner by nigerose of NOS. NOS enhanced interleukin 12 (IL-12) and interferon-gamma (IFN-gamma) production by normal splenocytes in the presence of the potent IL-12 inducer, heat-killed Lactobacillus plantarum L-137, in vitro. Consistent with the in vitro finding, L. plantarum L-137-induced IL-12 production and IL-2-induced IFN-gamma production were augmented in mice fed with a 14.6% NOS diet for 2 weeks compared with mice fed with a control diet. Notably, mice fed with the NOS diet showed significantly longer survival time than the control mice after the induction of an endogenous infection by administering 5-fluorouracil in a lethal dose. Taken together, these results suggest that NOS may exert immunopotentiating activity through the activation of an IL-12-dependent T helper 1-like immune response.     

Role of organ-associated NK cells in decreased formation of experimental metastases in lung and liver

Mice treated with anti-asialo GM1 (asGM1) serum exhibited increased formation of experimental metastases in lung and liver after i.v. challenge with B16 melanoma or Lewis lung carcinoma. This increased metastasis formation coincided with decreased splenic NK activity and increased survival of i.v. injected radiolabeled tumor cells. In contrast, the injection of mice with the pyran copolymer maleic anhydride divinyl ether (MVE-2) augmented NK activity in the spleen and significantly depressed the formation of experimental metastases in the lungs and liver. However, a single or double administration of anti-asGM1 antiserum to MVE-2-pretreated mice failed to inhibit the immunoprophylaxis associated with MVE-2 administration, although it did decrease splenic NK activity and also increased the survival of i.v.-injected radiolabeled tumor cells. To address the mechanism for this dichotomy, we examined NK activity not only in the spleen but also in the blood, lungs, and livers of MVE-2-treated mice. Levels of NK activity in the lungs and liver were several-fold higher than those observed in spleen and blood. However, MVE-2-augmented NK activity in lung and liver was more resistant to depletion by the standard regimen of anti-asGM1 treatment than was NK activity in blood and spleen, and required two high-dose administrations of a higher titered antiserum for depletion of the augmented response. This high-dose regimen removed all detectable NK activity from the lung and liver, and concomitantly eliminated the metastasis-inhibiting effect of MVE-2. These data are consistent with a role for organ-associated NK cells in inhibiting metastasis formation during the extravasation and/or early postextravasation phases of the metastatic process. The results also suggest that biologic effects of NK activity in spleen and blood can be dissociated from those mediated by NK activity in other organs by use of different treatment regimens with anti-asGM1 serum. Finally, because NK activity in target organs can be augmented to an even greater extent than in the blood and spleen by at least some biologic response modifiers (BRMs), organ-associated NK activity should be considered as a possible mechanism for the therapeutic effects of BRM treatment.     

Effect of levamisole on cytotoxic T-cell-mediated immune resistance to L1210 murine leukemia in hyperimmune mice

Summary The effect of levamisole (LMS) on T-cell-mediated antitumor immunity was examined in adult and aged mice hyperimmune to L1210 leukemia. The immune resistance of aged mice was depressed compared with that of adult mice, which almost completely rejected 5×10⁷ L1210 cells inoculated IP. A significant level of tumor-specific cytotoxicity was detected in the spleen cells of adult hyperimmune mice by the ⁵¹Cr-release assay after in vitro sensitization with mitomycin C-treated L1210 cells. This was mediated by cytotoxic T cells, since in vivo administration of antithymocyte serum or in vitro treatment of the spleen cells with anti-Thy 1.2 antibody and complement abrogated the cytotoxicity completely. In aged mice, however, cytotoxic T-cell activity was lower although the animals were immune to L1210.Administration of LMS (0.38 mg/kg) restored the depressed cytotoxicity of aged mice to the level seen in adult mice. Furthermore, in adult hyperimmune mice LMS augmented T-cell-mediated cytotoxic activity and restored the reduced cytotoxicity caused by in vivo administration of antithymocyte serum. These results indicate that LMS was effective in augmenting T-cell-mediated tumor immunity in immunologically competent or deficient hosts.     

Natural cytolytic activity in mice with natural or induced cellular defects. I. Differential ability of in vitro interleukin-2 addition to augment natural cytolytic function

The ability of in vitro addition of recombinant interleukin 2 (rIL-2) to differentially enhance natural cytotoxicity was assessed using cells from mice with natural and induced cellular defects. In vivo treatment with most immunosuppressive or cytoreductive agents, anti-asialo-GM1 antibody, or gamma irradiation dramatically reduced in vitro cytotoxicity against natural killer (NK) sensitive targets by direct reduction in either percentage specific lysis or lytic units per spleen. In most cases, in vitro addition of rIL-2 (at concentrations causing augmented NK function in cells from naive Balb/C mice) enhanced cytotoxic activity of cells from treatment groups to a normal value but not within the rIL-2-enhanced range of nontreated animals. Additionally, cytotoxic activity of cells from animals treated with certain drugs or gamma irradiation could be augmented by rIL-2 when measured by percentage lysis but not lytic units per spleen. In vivo treatment with cyclosporin A did not affect natural cytotoxic activity and addition of rIL-2 augmented the NK activity in a similar fashion to the profile of naive cells. In experiments using cells from beige (C57Bl/6-bg) mice which have a natural defect in NK activity against YAC-1 targets, addition of rIL-2 (at concentrations causing augmented natural cytotoxic function in cells from C57Bl/6 mice) could not effectively enhance in vitro natural cytotoxic function.     

Dual effects of OK-432 on mitogenic response of splenocytes to concanavalin A

OK-432, a streptococcal preparation, was studied for its effect on the concanavalin A (Con A)-induced mitogenesis of the host spleen cells. When mice were given a single intraperitoneal injection of OK-432, there was a substantial increase in the mitogenic response of splenocytes, whereas multiple injections conversely resulted in a marked reduction of the mitogenic response, when the spleen cells were cultured at high cell densities of over than 5 X 10(5) cells/well. The reduced Con A-responsiveness in the latter was not restored by mixing spleen cells from mice given multiple OK-432 injections with those from normal mice. Moreover, splenic macrophages from OK-432-injected mice exhibited marked inhibitory activity against Con A-mitogenesis of normal splenocytes, while normal splenic macrophages failed to show such an effect. Splenic T cells from OK-432-injected mice also showed an inhibitory activity against Con A-mitogenesis of normal splenocytes and similar activity was also noted in normal splenic T cells. Therefore, the OK-432-spleen cells contain two types of suppressor cells; one is a newly elicited suppressor macrophage and the other is a suppressor T cell supposedly resident also in normal spleen cells. In the OK-432-injected spleen cells, accessory cell function for T cell Con A-mitogenesis was markedly reduced. On the other hand, it was noted that the interleukin 2-producing ability of the OK-432-splenocytes was augmented more than that of normal splenocytes, indicating that multiple OK-432 injections also cause an increase in the helper T cell activity of the host spleen cells.     

Suppression of B-cell differentiation by natural killer (asialo GM1+) cells in mice

Mice were injected intravenously with rabbit antiserum to ganglio-n-tetraosylceramide (asialo GM1, ASGM1), a neutral glycosphingolipid present at high quantities on the surface of natural killer (NK) cells. Spleen cells prepared from the mice were then examined for NK activity against YAC-1 targets, for phagocytic cells and by flow cytometric analysis for Thy1, Lyt1, Lyt2, ASGM1 and surface Ig (SIg) phenotypes. Administration of anti-ASGM1 in mice resulted in a complete depletion of NK activity and ASGM+1 cells in the spleen, but no changes in the proportions of Thy1+ cells and their Lyt1+ and Lyt2+ subsets and phagocytic cells. Corresponding to this selective depletion of ASGM+1 cells and NK activity, the spleen cells showed an increased number of SIg+ B cells and augmented mitogenic responses to B-cell but not T-cell mitogens. These NK-depleted spleen cells also showed production of pokeweek mitogen (PWM)-driven plaque-forming cells (PFC) to much higher levels than those of control spleens. In the spleens of mice treated with varying concentrations of anti-ASGM+1, a good correlation was found between the decreased NK activity and the enhanced PFC response. To directly test the possible suppressor activity of NK cells on PWM-induced PFC response, NK (ASGM+1) cells were highly purified from the spleen by a combination of Percoll gradients and cytolysis of T cells by monoclonal antibodies followed by indirect panning. When added to NK-depleted spleen cells, they suppressed the augmented PFC response of NK-depleted spleen cells, depending on the number of cells added. These results suggest that NK (ASGM+1) cells in mice exhibit a suppressor property on B cells, which are undergoing spontaneous or mitogen-induced differentiation.     

Dietary restriction reduces the incidence of 3-methylcholanthrene-induced tumors in mice: Close correlation with its potentiating effect on host T cell functions

Summary All mice treated with 3-methylcholanthrene (MC) suffered with tumor 114 days after treatment. However, 40% dietary restriction caused a great inhibition of tumor incidence. In order to understand the mechanisms by which dietary restriction decreased the occurrence of tumor in mice, we investigated the correlation between tumor incidence and host T cell immune responses. At 114 days after MC administration, the mice were sacrificed and their T cell immune responses were assessed. Flow cytometry studies demonstrated that dietary restriction caused a marked increase of the proportion of Thy1.2⁺, L3T4⁺ T cells in MC-treated diet-restricted mice. Consistent with this result, T cell responses against concanavalin A and interleukin-2 were also potentiated in spleen cells obtained from MC-treated diet-restricted mice, while spleen cells obtained from MC-treated unrestricted mice showed decreased T cell responses because of their tumor burden. Such potentiation of T cell functions by dietary restriction was also observed at earlier stages of MC-induced tumorigenesis. During the course of carcinogenesis, spleen cells obtained from diet-restricted mice showed decreased natural killer activity in vivo. However, in vitro induction of cytotoxic T cells was markedly augmented in MC-treated diet-restricted mice compared with unrestricted mice. These results strongly suggest that the increase of host T cell immune responses might be one of the major causes for the reduction of tumor occurrence by dietary restriction.     

双歧杆菌和乳杆菌在诱发抗肿瘤免疫中的作用

双歧杆菌和乳杆菌给封闭群昆明小鼠腹腔注射,在体内激活后,胸腺细胞和脾细胞对ＣｏｎＡ刺激的增殖反应,脾贴附性细胞对ＹＡＣ－１,Ｌ９２９的细胞毒作用,以及脾贴附性细胞产生对上述二株瘤细胞的肿瘤坏死因子（ＴＮＦ）的活性都比对照动物明显增强。结果提示短双歧杆菌和嗜酸性乳杆菌给小鼠腹腔注射后,通过激活脾脏淋巴细胞和贴附性细胞（巨噬细胞）所介导的免疫功能而明显地增强宿主的抗肿瘤活性。     

Effective inhibition of Candida albicans growth by the combination of murine peritoneal neutrophils and activated macrophages.

The effect of leukocytes on the anti-Candida activity of neutrophils was examined. Murine neutrophils which were purified from casein-induced peritoneal cells inhibited the mycelial growth of Candida albicans. This anti-Candida activity of neutrophils was augmented by the addition of spleen cells prepared from mice pretreated with bacterial lipopolysaccharide 3 hr before, but not from non-treated mice. The population in the spleen cells, which enhanced the anti-Candida activity of neutrophils, was plastic-plate adherent, nylon-fiber columns adherent and anti-Mac-1 antigen-positive. These immunological profiles suggested that the enhancing cells are classified to splenic macrophages. Peritoneal-exudated macrophages from mice treated with lipopolysaccharide also augmented the anti-Candida activity of neutrophils. These results suggest that the anti-Candida activity of neutrophils may be upregulated by activated macrophages.     

Augmentation of mouse liver-associated natural killer activity by biologic response modifiers occurs largely via rapid recruitment of large granular lymphocytes from the bone marrow

A variety of biologic response modifiers (BRM) can potently augment NK activity in nonlymphoid organs. By using the liver as a model organ, we have shown that this augmentation of organ-associated NK activity is coincident with a 10- to 15-fold increase in the number of large granular lymphocytes (LGL) which can be isolated. The present study was designed to investigate the mechanism by which BRM induce this increase in liver-associated LGL and the coincident increase in hepatic NK activity. Initial studies confirm that a single dose of the pyran copolymer, maleic anhydride divinyl ether (MVE-2), augmented hepatic NK activity and increased the number of liver-associated LGL from 3 x 10(4)/liver to 5 x 10(5)/liver (a 17-fold increase). Multiple injections of MVE-2 further augmented total liver-associated NK activity and LGL number (to 13 x 10(5)/liver). As expected, both the NK activity and detectable LGL were eliminated by treatment of the mice with antiasialo GM1 (asGM1) serum. Three possible mechanisms for the BRM-induced increase in liver-associated LGL have been investigated, including 1) the rapid proliferation of resident hepatic LGL, 2) the redistribution of mature LGL from peripheral sites such as the spleen, or by 3) a rapid output and subsequent hepatic localization of LGL or their precursors recently derived from the bone marrow (BM). Our results demonstrated that the contribution of in situ proliferation to the BRM-induced increase in liver-LGL was relatively small, since the number of cells expressing NK-associated markers (i.e., asGM1, Thy-1.2, and NK1.1) and in G2/M phase (as assessed by propidium iodide uptake) was only 4 to 8%. Further experiments demonstrated that splenectomy before the administration of MVE-2 did not inhibit the augmentation of liver-associated NK activity. This result argued against a recruitment of mature LGL from the spleen. In contrast, selective depletion of the BM following administration of 89Sr decreased the ability of MVE-2 to augment liver-associated NK activity by greater than 80%. This procedure also significantly decreased the ability of Propionibacterium acnes (85%) and multiple doses of IL-2 (49%) to augment liver-associated NK activity. These results demonstrate that the rapid augmentation of liver-associated NK activity by BRM is largely due to localization and accumulation in the liver of LGL recently derived from the BM.     

Antigen-specific augmentation factor involved in murine delayed-type footpad reaction. IV. Effect of delayed-type hypersensitivity augmentation factor on in vitro induction of DTH

We found an antigen-specific factor capable of augmenting delayed-type hypersensitivity (DTH) in the serum of mice sensitized with heterologous erythrocytes to induce a delayed footpad reaction (DFR), or in the culture supernatant of the mixture of sensitized T cells and specific antigens. This factor (DTH augmentation factor; DAF) was confirmed to augment DTH in transferred recipients. In this paper, such an activity of DAF was further investigated using the system with in vitro induction and local transfer of DTH. DAF also augmented the primary in vitro induction of DTH, when spleen cells from mice transferred with the DAF-containing serum 12 hr previously or spleen cells incubated with the DAF-containing serum on ice for 2 hr were cultured with heterologous erythrocytes. DAF acted on the induction phase of DTH and augmented a typical DTH which was dependent on Thy-1-positive T cells. DAF showed antigen specificity, but was not assigned to conventional immunoglobulin. The activity of DAF was detected when nylon-wool nonadherent cells were incubated with DAF prior to the culture of those cells and antigens, but not detected when only nylon-wool adherent cells were incubated with DAF. Thus, DAF exerted its effect through binding to acceptor cells which were included in nylon-wool nonadherent spleen cells from normal mice.     

OK-432 stimulates primary production and activity of murine natural killer cells

Although many immunostimulants have been shown to increase the lytic activity of natural killer (NK) cells in the periphery, little is known about their effects on NK cells in the bone marrow, the primary site of NK production. In the experiments reported here, we tested OK-432, a pharmaceutical preparation of Streptococcus pyogenes, for its effects on both the primary production and lytic activity of NK cells in C57BL/6J mice. NK activity in bone marrow cells (BMC) and spleen cells (SC) was significantly increased following intravenous administration of OK-432, peaking on day 2 in BMC and on day 3 in SC. Concomitantly, there were marked changes in the cellularity in the two compartments. Bone marrow cellularity fell significantly on day 1 post-OK-432 and then gradually returned to normal, whereas spleen cellularity rose rapidly and remained elevated. As a consequence, the total NK activity (per femur or per spleen) was significantly increased at 48-96 h after administration of OK-432. The target specificity was unchanged. The phenotype of NK cells in BMC as determined by cytotoxic depletion was unchanged by OK-432, but splenic NK activity shifted to a 'less mature' phenotype, intermediate between that of normal BMC and SC. Cytokinetic studies using 3H-TdR revealed an increase in the production of NK cells in the bone marrow following administration of OK-432. Proliferating NK cells also appeared in the spleen. Whether these were recently produced NK cells from the bone marrow that still retained the ability to proliferate or mature NK cells that were stimulated into cell cycle cannot be determined from these experiments. These data are the first to directly demonstrate the modulation of the primary production of NK cells by an immunologically active drug.     

Augmentation of Mouse Natural Killer Cell Activity by Lactobacillus casei and Its Surface Antigens

Lactobacillus casei YIT 9018 (LC 9018) augmented the natural killer (NK) cell activity of spleen cells from inbred BALB/c mice injected intravenously with LC 9018 or intraperitoneally with polyinosinate-polycytidylate. Augmentation of this activity by LC 9018 was also observed in male C3H/He, CBA/N, and C57BL/6 mice. The spleen cells exhibited no cytolytic activity against P815, a cell line insensitive to NK cells. The cytolytic activity of the spleen cells increased 2 days after the injection of 250 μg of LC 9018/mouse, peaked on day 3, and gradually declined thereafter. The increase caused by LC 9018 was also observed in normal and Meth A-bearing mice. In vitro treatment with anti-asialo GM1 antibody plus complement completely-abrogated the LC 9018-augmented murine NK cell activity. The NK activity on the 3rd day after LC 9018 injection was reduced by in vitro treatment with anti-Thy 1.2 monoclonal antibody plus complement to half of that observed when treatment was with complement alone. This suggests that there were two populations of NK cells in the spleen cell suspension derived from LC 9018-treated mice. One population was asialo GM1-positive and Thy 1-negative, the other was asialo GM1-positive and Thy 1-positive.     

Induction of interferon-gamma in mouse spleen cells by OK-432, a preparation of Streptococcus pyogenes

A bacterial antitumor and immunopotentiating agent, OK-432, induced Interferon in the spleen cell cultures but not in the thymus cell cultures of various inbred strains of mice. When 1 × 10⁷ spleen cells were cultured in the presence of 5 μg/ml of OK-432, interferon activity was detected as early as 4 hr later and reached a maximum level of about 160 to 500 units/ ml 24 hr later. OK-432-induced interferon was mainly an IFN-γ of molecular weight approximately 40,000, but also contained IFN-α and IFN-β.     

Development of hyporesponsiveness to augmentation of natural killer cell activity after multiple doses of maleic anhydride divinyl ether: association with decreased numbers of large granular lymphocytes

A single injection of mice with maleic anhydride divinyl ether (MVE-2) resulted in significantly augmented natural killer (NK) activity. However, multiple injections with MVE-2 led to a hyporesponsive to boosting of NK activity. Stimulation of prostaglandin E secretion of induction of suppressor macrophages (M phi) or lymphocytes were shown not to be responsible for the depressed NK cell responses. Rather the hyporesponsiveness to NK boosting was associated with a decreased number of large granular lymphocytes (LGLs). Percoll discontinuous-density-gradient studies showed that the augmented NK activity of spleen cells after a single injection with MVE-2 was associated with an increase in the percentage of LGLs in the lower-density fractions (Fraction 1 and 2). In contrast, the NK activity and percentage of LGLs in the lower-density fractions were markedly decreased after multiple injections with MVE-2. Polyinosinic: polycytidylic acid stabilized with poly(L-lysine) in carboxymethylcellulose, another BRM capable of augmenting NK activity, was able to substantially augment NK activity in mice hyporesponsive to MVE-2 and this was accompanied by an increase in the percentage of LGLs in the lower-density Percoll fractions.     

T-cell responses induced by the parenteral injection of antigen-modified syngeneic cells. III. Dissociation of primed cytolytic T-cell and efferent suppressor-T-cell activity following intravenous injection of trinitrophenol-modified syngeneic spleen cells

The parenteral injection of ligand-coupled syngeneic spleen cells has profound effects on immune responsiveness. In this regard, it was examined whether the primed in vitro trinitrophenol (TNP)-specific cytotoxic T-lymphocyte (CTL) responses observed in splenic T-cell populations from mice injected intravenously (iv) with syngeneic TNP-modified spleen cells (TNP-SC) are related to the efferent-acting suppressor-T-cell (Ts) activity observed in splenocytes from iv primed mice. Treatment of mice with cyclophosphamide, adult thymectomy, or monoclonal anti-I-J antiserum prior to the iv injection of TNP-SC was found to eliminate the ability of splenic Ts from these mice to suppress the passive transfer of delayed-type hypersensitivity (DTH) mediated by trinitrochlorobenzene-immune T cells. In contrast, spleen cells from these pretreated mice showed no impairment in the development of augmented TNP-specific CTL responses upon in vitro restimulation with TNP-SC. Separation of the two activities was also achieved in a kinetic analysis. It is concluded that specific enhancement of CTL responsiveness induced by the iv injection of TNP-SC is related to the expansion of a population prelytic Lyt 2+ CTL effector cells which does not appear to contain efferent-acting Lyt 2+ Ts active in suppressing DTH expression.     

Changes in number and density of large granular lymphocytes upon in vivo augmentation of mouse natural killer activity

NK activity of mice as well as humans and rats has been clearly associated with large granular lymphocytes (LGL). To better understand the effects of interferon (IFN) and IFN inducers on natural killer (NK) cells, we have compared the LGL in the spleens of normal and boosted mice. Cells were fractionated by centrifugation on discontinuous Percoll density gradients, and each fraction was tested for NK activity against YAC-1 targets and for the presence of LGL. In vivo treatment with C. parvum (0.7 mg/mouse, i.p., day-3), MVE-2 (25 mg/kg, i.p., day-3), poly I:C (4 mg/kg, i.p., day-3), or IFN (10(5) U/mouse, i.p., day-1) resulted in a marked augmentation and a change of distribution of cytotoxic activity. Most of the NK activity of boosted spleen cells was associated with lower density fractions 1 and 2, whereas active normal spleen cells had somewhat higher density (fractions 2 and 3). In parallel to their increased reactivity, the boosted spleens had a marked increase in the percentage of LGL, particularly in fractions 1 and 2. The augmented activity appeared to be mediated by the LGL, because treatment with anti-asialo GM1 or anti-Thy-1.2 plus complement reduced NK as well as the number of LGL. These results indicate that IFN-mediated boosting of NK activity in the spleen is due to an increase in the lower density LGL, as well as to an increase in the function of preexisting NK cells.     

Participation of hematopoietic stem cells in the process of immune response formation

The endogenous colony formation and immunogenesis in mice of CBA and C57BL strains immunized with various doses of sheep red blood cells (SRBC) were studied. An absolute or relative increase of antibody forming cells under the action of growing SRBC dose from 2.10(7) to 2.10(8) as well as a decrease following 2.10(9) dose administration were noted. The number of endogenous hemopoietic spleen colonies augmented depending on increased antigen dose. Strain differences in the number of endogenous spleen colonies remained. It is suggested that the stimulation mode of hemopoietic lines is based on a specific RES blockade by SRBC resulting in enhanced proliferative effect of stem cells.