Molecular basis of "suppressor" macrophages. Arginine metabolism via the nitric oxide synthetase pathway.

A molecular explanation for "suppressor" macrophage inhibition of lymphocyte proliferation is described. NG-monomethyl-L-arginine (NGMMA), a specific inhibitor of the nitric oxide synthetase pathway, markedly augments Con A-induced proliferation of rat splenic leukocytes. Macrophages are necessary and sufficient for NGMMA-releasable-suppression, as indicated by a loss of suppression after either pretreatment of isolated splenic macrophages with NGMMA or their depletion by plastic adherence or L-leucine methyl ester. L- (but not D-) arginine overrides NGMMA-releasable suppression, and suppression is blocked by RBC as would be expected if nitric oxide were the effector molecule. Unlike rats, NGMMA did not augment Con A-induced proliferation of normal mouse splenic leukocytes. However, NGMMA did augment Con A-induced proliferation of mouse splenic leukocytes induced to contain suppressor macrophages by intravenous injection of Corynebacterium parvum, which suggests a quantitative, not qualitative, difference in suppressor macrophages between rats and mice. Nitrite production, as an indicator of nitric oxide synthesis, correlated with suppressor macrophage activity in rats and mice and was inhibited by NGMMA. Finally, NGMMA also markedly enhanced proliferation with every other mitogen examined (PHA, protein A, PWM, and LPS). It is concluded that immunoregulation of lymphocyte proliferation by suppressor macrophages is mediated, in part, directly or indirectly by products of the nitric oxide synthetase pathway.     

Suppression of lymphokine production: II. Macrophage-dependent inhibition of production of macrophage activating factor

The ability of different populations of macrophages to affect the production of macrophage activating factor (MAF) by stimulated T lymphocytes was investigated. We found that activated macrophages, infiltrating MSV-induced regressing tumors or macrophages recovered from the peritoneum of mice injected with Corynebacterium parvum, were able to actively suppress the production of MAF. MAF production by antigen-stimulated MSV-immune or -alloimmune spleen cells and by normal spleen cells stimulated by Con A was susceptible to macrophage-dependent suppression to a similar extent. In contrast, resident macrophages or those elicited by light mineral oil or proteose-peptone did not affect MAF production. While suppressor macrophages added at the time of the lymphocyte stimulation inhibited MAF production, the same cells added 4–6 hr after stimulation were ineffective. Therefore, it seems that the macrophages suppressed the early events of lymphocyte activation leading to MAF production. Suppressor macrophages, by inhibiting MAF production, may limit the expansion of the cytotoxic activity. This regulation of macrophage functions, mediated by the effects of suppressor macrophages on T lymphocytes, could be responsible for an insufficient antitumor cytotoxic response by macrophages.     

Immunoregulatory activity of culture-induced suppressor macrophages

Rat splenic cells precultured in vitro for 5 days exhibited marked suppressive activity on the secondary cytotoxic T lymphocyte (CTL) response to a Gross virus-induced lymphoma. Suppressive activity was produced by macrophages (MØ) rather than lymphocytes and as low as 1% MØ content was sufficient to achieve completely inhibited CTL responses. Aspirin, indomethacin, and d,l-6-chloro-2-methylcarbazole-2-acetic acid prevented cultured splenic MØ from exerting their inhibitory effect, thereby suggesting a role for prostaglandins in suppression. Events which occurred within the first 24 to 48 hr of the CTL response were susceptible to the suppressive action of MØ since normal CTL responses were obtained if suppressive MØ were added later than Day 2 or if indomethacin was added within the first 24 to 48 hr of culture. Two processes of lymphocyte activation, namely blast transformation and DNA synthesis, were inhibited in the presence of suppressive MØ. However, suppression of these processes did not result in the loss of CTL progenitor cells since CTL responses that were inhibited in the presence of suppressive MØ proceeded normally following their removal.     

Studies on the regulation of lymphocyte reactivity by normal and activated macrophages.

To determine the potential role of macrophages as regulators of the immune response, the effect of mouse peritoneal macrophages on transforming mouse spleen lymphocytes was investigated. Mitogen and antigen stimulated lymphocyte transformation, as measured by DNA synthesis, was enhanced by all concentrations of normal macrophages tested, but only by low concentrations of activated macrophages. High concentrations of activated macrophages markedly inhibited lymphocyte transformation. This inhibition occurred whether lymphocyte DNA synthesis was measured by incorporation of [³H]TdR or of ³²P. Activated macrophages cultured with lymphocytes within 4 hr of being removed from the peritoneal cavity inhibited lymphocyte transformation. When activated macrophages were cultured alone for 24 or more hours before addition of lymphocytes, enhancement of transformation was noted. Once lymphocytes were exposed to activated macrophages, they could not be induced to undergo transformation in the presence of Con A. Whereas heat-killed activated macrophages, which appeared intact morphologically, lost their capacity to inhibit lymphocyte transformation, macrophages treated with mitomycin C to inhibit DNA synthesis retained this capacity. Syngeneic and allogeneic macrophages had similar inhibitory ability. Supernatants from cultures of many cell types (including normal or activated macrophages, lymphocytes, lymphocytes plus macrophages, and L cells) inhibited [³H]TdR incorporation by both mitogen stimulated lymphocytes and tumor cells. These studies demonstrate the capacity of macrophages to regulate lymphocyte transformation in vitro and suggest a role for these cells as regulators of cell-mediated immunity in vivo.     

Further studies of the selective inhibition by ouabain of antigen-induced proliferation of human lymphocytes

Cultures of human lymphocytes incubated for 48 hr in the presence of 2 × 10^?7M solutions of the cardiotonic steroid ouabain lose the proliferative response to antigens (SL-0, SK-SD) but can still proliferate when stimulated by nonspecific mitogens (PHA, Con A, pokeweed mitogen). The two-way mixed lymphocyte reaction was also irreversibly lost if cells of both donors were subjected to ouabain pretreatment. Neither cell counts nor cell viability (determined by dye exclusion) were significantly affected by the ouabain treatment. Pretreatment of a suspension of macrophages with the cardiac glycoside did not diminish their capacity to restore the proliferative response to antigen of macrophage-depleted lymphocyte suspensions; on the other hand, untreated macrophages could not restore the proliferative response of cultures of ouabain-pretreated lymphocytes. The ouabain treatment did not change the proportion of cells able to bind fluorescent anti-immunoglobulin nor did it modify the proportion of lymphocytes forming rosettes with either untreated, or antibody coated, red cells. Increased concentration of K⁺ in the medium, either during or after the ouabain treatment, did not reduce the ouabain effect. We conclude that the selective loss of certain lymphocyte functions caused by ouabain pretreatment was due to an effect on the lymphocyte and not on the macrophage; the effect was not due to the elimination of a relatively large fraction of the cells nor to a generalized disappearance of membrane antigens and receptors.     

Murine alveolar macrophage-mediated lymphocyte cytostasis: Kinetics and mechanisms

Recent reports have demonstrated that alveolar macrophages (AM) from several species regulate antigen- and mitogen-induced blastogenesis. In this study, we confirm that murine AM also mediate lymphocyte cytostasis and define, in part, the mechanism involved. AM were found to inhibit homologous splenocyte responses to concanavalin A in a dose-dependent manner. The inclusion of 1 AM:10 lymphocytes abrogated mitogenesis. Kinetic studies revealed that maximal inhibition of the splenocyte response required the inclusion of AM at culture initiation, stimulation of splenocytes with an optimal Con A dose, and an optimal incubation period of 72 hr. In addition, suppression of Con A-induced blastogenesis by AM was not genetically restricted, as Balb/c AM suppressed allogeneic CBA/J spleen cells comparably to homologous control cells. The addition of either catalase or indomethacin to partially suppressed cultures (containing 3% AM) totally reversed the inhibition. In contrast, catalase did not protect lymphocytes from absolute suppression mediated by higher AM numbers (10% AM), while indomethacin offered partial protection. A synergistic effect was noted upon the addition of both substances. Thus, prostaglandin and hydrogen peroxide released by AM contribute to the suppressive effects of these cells.     

应激抑制淋巴细胞转化的时间效应

本研究吵缚方法使大鼠接受应激刺激,然后分别取大鼠应激３、６、１２、１８ｈ和解除束缚后１２、２４、４８、７２、９６ｈ的淋巴结、脾脏提取物和血清,与刀豆素Ａ同时加入正常大鼠肠系膜淋巴结细胞悬液中育７２ｈ后用噻唑蓝（ＭＴＴ）比色分析法检测肠系膜淋巴结细胞的转化,来应激抑制淋巴细胞转化作用的出现和消失过程。结果如下：（１）应激３和６ｈ大鼠的淋巴结、脾脏提取物和血清对淋巴细胞的转化都没有明显的影响;（２）应     

Modulation of Epstein-Barr virus-induced B-cell activation by concanavalin A

Direct addition of the T-cell mitogen, concanavalin A (Con A), to cultures of Epstein-Barr virus (EBV)-stimulated peripheral blood mononuclear cells (PBMC) resulted in a dose-dependent inhibition of immunoglobulin M (IgM) secreted in the supernatant, as measured by an enzyme-linked immunosorbent assay. Furthermore, Con A inhibited IgM secretion of isolated T-depleted cells stimulated with EBV, and both the proliferation and IgM secretion of EBV-driven lymphoblastoid cell lines. T-Enriched cells, precultured for 48 hr with Con A, were also able to suppress the IgM response of fresh autologous PBMC stimulated with EBV. This suppression was radiation sensitive (2000 rad), a procedure which resulted in enhancement of the IgM secretion of the responder cells in two out of three experiments. Studies on the long-term effects of Con A showed that the early suppression of IgM secretion was transient and that the mitogen prevented the development of the cytotoxic T-cell response normally seen with lymphocytes from EBV-seropositive donors after 5 weeks of culture. Thus, Con A appears to modulate human lymphocyte responses to EBV by multiple mechanisms.     

Suppression of the in vitro secondary antibody response of rabbit lymphoid cells by Concanavalin A.

The effect of various concentrations of concanavalin A (Con A) on the in vitro secondary antibody response of rabbit lymph node and spleen cells to sheep red blood cells (SRBC) was studied. Complete suppression of the IgM plaque-forming cell (PFC) response of both lymph node and spleen cultures was observed when 10 mug/ml of Con A was added at the time of initiation of the cultures whereas only partial suppression was observed when 1 mug/ml of Con A was added. Moreover, marked suppression of the immune responses of both spleen and lymph node cultures was observed when 10 mug/ml of Con A was added at 24 hr after antigenic challenge and to a lesser extent when added at 48 hr. Suppression of the IgM PFC response was also detected when spleen cultures were exposed to 10 mug/ml of Con A for as little as 2 hr after antigenic challenge. However, substantial increases in DNA synthesis were observed only in those cultures which were in contact with Con A for at least 24 hr. Finally evidence is presented that the Con A-induced suppression is mediated by a soluble substance(s).     

Adenosine deaminase and immunodeficiency: An in vitro model

We have examined the effect of adenosine and EHNA, a competitive inhibitor of adenosine deaminase (ADA), upon the ability of human peripheral blood lymphocytes to respond to mitogen. Addition of adenosine at concentrations greater than 10 μm (10^?5m) resulted in inhibition of lymphocyte proliferation at 48 hr of culture, provided that the culture medium was relatively free of ADA activity. The actual concentrations of adenosine remaining in inhibited cultures at the time of harvest were considerably lower than those added initially. EHNA alone also inhibited PHA response (and to a lesser extent PWM and Con A responses), but only at high concentrations. Noninhibitory concentrations of EHNA and adenosine together acted synergistically to produce profound inhibition of lymphocyte proliferation. This may provide an in vitro model to explore further the mechanism of the immunodeficiency associated with deficiency of ADA. Adenosine deaminase activity in stimulated cultures did not differ significantly from that found in unstimulated cultures, and the activity per protein or per DNA actually decreased in stimulated versus unstimulated cultures.     

Modulation of lymphocyte proliferation by macrophages and macrophages loaded with arachidonic acid

Arachidonic acid (AA) is incorporated and exported by macrophages. This fatty acid is also transferred from macrophages (Mphi) to lymphocytes (LY) in co-culture. This observation led us to investigate the effect of macrophages pre-loaded with AA on concanavalin A (Con A)-stimulated lymphocyte proliferation. The experiments were performed in co-culture. This condition reproduces the in vivo microenvironment in which the modulation of lymphocyte proliferation is dependent on the interaction with macrophages. Lymphocytes obtained from untreated rats or from intraperitoneally thioglycolate-injected rats (THIO-treated) were co-cultured with macrophages from the same rats. Firstly, macrophages were co-cultured for 48 h with Con A-stimulated lymphocytes in different proportions: 0.5, 1, 2.5, 5, 10, 20 and 30% of 5 x 10(5) lymphocytes per well. At 1% proportion, macrophages caused maximum stimulation of lymphocyte proliferation; a four- to five-fold increase, for cells from both thioglycolate-treated and untreated rats, respectively, whereas at 20% it caused maximum inhibition. In addition, 1 or 20% macrophages were pre-loaded with several AA concentrations during a period of 6 h and co-cultured with lymphocytes. At 180 microM AA and 1% macrophages, lymphocyte proliferation was inhibited (by 25%), whereas at 20% macrophages, proliferation was increased, by 25- and three-fold, respectively, for cells from untreated and THIO-treated rats. AA added directly to the medium reduced lymphocyte proliferation, also being toxic to these cells at 100 microM. No toxic effects of AA were observed on macrophages. Additional evidence suggests that nitric oxide production is involved in the modulation of lymphocyte proliferation by AA-pre-loaded macrophages. These findings support the proposition that AA can directly modulate lymphocyte proliferation and the interaction between macrophages and lymphocytes.     

A differential effect of IgM and IgG antibodies on the blastogenic response of lymphocytes to rubella virus

Peripheral blood lymphocytes of rabbits immunized with live rubella vaccine respond to rubella virus antigens in tissue culture with increased DNA synthesis as measured by incorporation of ³H-thymidine. This reaction can be inhibited by rubella antibody. A dose dependent effect was observed when antibodies in whole serum were mixed with virus prior to addition to lymphocyte cultures. When antisera were fractionated and their individual immunoglobulins tested, a paradoxical effect was obtained. Immune IgG although it was highly effective in neutralizing the virus was incapable of inhibiting the lymphocyte response and at times caused an increased response. In contrast, immune IgM which was less efficient in neutralizing virus caused significant suppression of the blastogenic reaction. By themselves these results might have signified that IgG and IgM antibodies have different specificities or different binding properties with respect to viral surface antigens. However, immune complexes consisting of virus and IgM reduced response of both rubella immune and normal rabbit lymphocytes to PHA. This nonspecific inhibitory action required a specific step of antigen and IgM antibody interaction and normal IgM-virus mixtures or mixtures of anti-rubella IgM and poliovirus or influenza virus did not suppress lymphocyte response to PHA. Anti-rubella IgG complexed with rubella virus did not suppress the PHA response. The IgG function was apparently limited to neutralization of the infectivity of rubella virus whereas the major role of IgM was manifested through its suppressive effect on lymphocyte reactions.     

Suppression of lymphokine production: I. Macrophage-mediated inhibition of MIF production

Macrophages have been found to suppress the in vitro production by stimulated T lymphocytes of a lymphokine, migration inhibitory factor. When macrophages isolated from primary MSV-induced tumors were added to antigen-stimulated MSV-immune spleen cells, a complete suppression of MIF production was observed. This suppression was nonspecific, since MIF production by antigen-stimulated alloimmune spleen cells and by PHA-stimulated normal spleen cells was also inhibited. Suppressor macrophages could also be induced by inoculation with Corynebacterium parvum, whereas light mineral oil-induced peritoneal macrophages had no detectable effect on MIF production. The failure to detect MIF in the supernatants of stimulated cultures containing activated macrophages appeared to be due to inhibition of lymphokine production rather than to absorption or inactivation of MIF or to interference with the assay for detection of MIF. Macrophages were able to suppress MIF production only when added during the first 4–5 hr of culture and they had no effect when added later. These data show that activated macrophages can nonspecifically suppress lymphokine production and that this appears to be due to inhibition of an early step in lymphocyte stimulation.     

Antibodies to nonpolymorphic determinants of the Thy-1 molecule inhibit T cell proliferation

The effect of anti-Thy-1 monoclonal antibodies on murine mixed lymphocyte reactions and concanavalin A-induced mitogenesis were investigated. It is demonstrated that rat antibodies against nonpolymorphic determinants of the murine Thy-1 antigen inhibited cell proliferation in the absence of complement. In contrast, antibodies against polymorphic determinants of Thy-1 had no effect on T cell activation. Inhibition of T cell proliferation did not depend on the isotype of the blocking antibody, because both IgM and IgG antibodies against monomorphic determinants were inhibitory, whereas IgM or IgG antibodies against allotypic determinants were inactive. In addition, the blocking activity could not be attributed to the xenogeneic (rat) origin of the antibodies to nonpolymorphic Thy-1 determinants, because rat anti-Thy-1.2 antibodies had no effect on cell activation. Thus, the efficacy of anti-Thy-1 antibodies as T cell inhibitors was determined by the antibody specificity. The suppressive mechanism of anti-Thy-1 antibodies was effective throughout the entire course of mixed lymphocyte reactions. Addition of antibodies at any time point during the first 90 hr of a 120-hr mixed lymphocyte culture resulted in significant suppression of the proliferative response. However, in some cases an early enhancement preceded suppression of the response. The modulation of proliferative responses by anti-Thy-1 did not result from a nonspecific mitogenic effect of the antibodies on T lymphocytes, because no effects were observed when antibodies were added to responder cells alone. These results suggest that the Thy-1 molecule, or a molecule that is located on the cell membrane in close proximity to the Thy-1 antigen, is involved in the activation of T lymphocytes.     

Amphotericin B both inhibits and enhances T-cell proliferation: inhibitory effect is mediated through H(2)O(2) production via cyclooxygenase pathway by macrophages

Amphotericin B (AmB) has been shown to have both immunosuppressive and -enhancing effects, making its precise nature of action enigmatic. In the present study, we found that AmB inhibited concanavalin A (Con A)-induced T cell proliferation if added within first 30 min of stimulation, after which inhibition began to diminish rapidly. However, AmB did not inhibit T-cell proliferation induced by a combination of PMA and ionomycin. AmB inhibition of Con A-induced proliferation was completely overcome by cyclooxygenase inhibitor ibuprofen ([alpha-methyl-4-(isobutyl)phenylacetic acid]) and H(2)O(2) scavenger catalase. In fact, in the presence of ibuprofen and catalase, AmB enhanced, instead of suppressing, Con A-induced proliferation in a dose-dependent way. The effect of catalase was limited to the removal of extracellular H(2)O(2) only, as the enzyme did not enter the cells. AmB stimulated H(2)O(2) production by macrophages, but not by a lymphocyte population, which was inhibited by ibuprofen. Our T-cell preparation contained about 3% macrophages, and AmB inhibition of proliferation was further pronounced by increasing the macrophage number by as little as 1%. Finally, AmB inhibition of Con A-induced T-cell proliferation was completely overcome by 2-mercaptoethanol. On the basis of these results, we suggest that AmB stimulates H(2)O(2) production by macrophages through the activation of the cyclooxygenase pathway of arachidonate metabolism. H(2)O(2) then inhibits Con A-induced T-cell proliferation by interfering with an early step of the T-cell receptor signaling pathway through the oxidative modification of some signaling proteins. Our results also show that AmB enhances T-cell proliferation, which can be seen only after blocking its inhibitory effect.     

Novel mechanism for Trypanosoma cruzi-induced suppression of human lymphocytes. Inhibition of IL-2 receptor expression

Co-culture of blood forms of Trypanosoma cruzi, the causative agent of Chagas' disease, with human PBMC impaired the capacity of T lymphocytes to express surface receptors for IL-2. This effect was evidenced by marked reductions in both the proportion of Tac+ cells and the density of Tac Ag on the surface of the positive cells, determined by flow cytometry. The extent of the inhibition increased with parasite concentration. Under optimal or suboptimal conditions of stimulation with either PHA or monoclonal anti-CD3, specific for an epitope of the T3-Ti human T cell Ag receptor complex, the presence of T. cruzi curtailed the capacity of T lymphocytes to proliferate and express Il-2R but did not affect IL-2 production. Furthermore, the addition of exogenous IL-2 did not restore the responsiveness of suppressed human lymphocytes but did when mouse lymphocytes were used instead. Therefore, unlike mouse lymphocytes, human lymphocyte suppression by T. cruzi did not involve deficient IL-2 production and was accompanied by impaired IL-2 utilization. Co-culture of human monocytes/macrophages with suppressive concentrations of T. cruzi increased IL-1 production, and the parasite did not decrease IL-1 secretion stimulated by a bacterial LPS. Therefore, the suppression of IL-2R expression and lymphoproliferation is not likely to have been an indirect consequence of insufficient IL-1 production due to infection of monocytes or macrophages. We have shown that suppression of human lymphocyte proliferation by T. cruzi is not caused by nutrient consumption, absorption of IL-2, lymphocyte killing, or mitogen removal by the parasite. Therefore, these results uncover a novel suppressive mechanism induced by T. cruzi, involving inhibited expression of IL-2R after lymphocyte activation and rendering T cells unable to receive the IL-2 signal required for continuation of their cell cycle and mounting effective immune responses.     

Activation and mechanism of action of suppressor macrophages

Intravenous administration of Corynebacterium parvum to alloimmunized mice activates splenic suppressor macrophages that effectively curtail primary and secondary generation of cytotoxic T lymphocytes (CTLs) in vitro. CTL generation was significantly inhibited in suppressed primary cultures by Day 3, the earliest time point that activity is first detected in control cultures. Suppressor macrophages had to be present during the first 24–48 hr of culture to effectively curtail the generation of CTLs. However, if suppressor macrophages were reactivated by 48-hr in vitro culture and then added to primary sensitizations that had been initiated 48 hr previously, they were capable of significant suppression. Suppressor cells produced a soluble factor that mediated the inhibition of CTL generation. The production or action of this factor could not be counteracted by indomethacin.     

Soluble suppressor activity of concanavalin A-activated spleen cells on B-lymphocyte colony formation in vitro.

Supernates from concanavalin A (Con A)-activated mouse spleen cell cultures suppress the formation of B-lymphocyte colonies (BLC) in soft agar culture by 30 to 95%. Con A-induced BLC suppressive culture supernates can be heated at 80 °C for 1 hr without losing activity. The BLC suppressive activity is eliminated totally by trypsin treatment and partly by treatment with β-galactosidase. Activity is unaffected by treatment with DNAse, RNAse, and α-glucosidase. By ultrafiltration the BLC suppressive factor(s) was shown to have a molecular weight greater than 300,000. These data suggest that BLC suppression is mediated by a protein-carbohydrate complex. BLC suppression was obtained when normal spleen cells were preincubated in Con A-activated supernates for only 1 hr at 37 °C. BLC suppressor activity was absent in the supernatant fluid of Con A exposed anti-θ-treated spleen cells, nonadherent spleen cells, extensively washed spleen cells, and spleen cells from nude (athymic) mice suggesting that cells responsible for Con A-induced BLC suppression are adherent, fragile cells of the T lineage. Con A-activated spleen cell supernates do not suppress colony formation in soft agar by normal mouse granulocyte-macrophage precursors, by plasmacytoma cells, T-lymphoma cells, or by carcinoma cells. However, colony formation by Abelson's murine leukemia virus transformed B-lymphoma cells was suppressed by 95% suggesting a relationship between this immature B-lymphoma line and B-lymphocyte colony-forming cells. Con A-activated spleen cell supernates do not suppress lymphocyte activation in liquid culture by phytohemagglutinin, Con A, or lipopolysaccharide. Heat-treated supernates—which inhibited BLC development by 90–95%—did not suppress the plaque formation by spleen cells immunized in vivo or in vitro by sheep red blood cells.     

Measles virus-induced suppression of lymphocyte reactivity in vitro

Measles virus (Edmonston strain B), in various multiplicities of infection, was added to human lymphocytes which were cultured in medium containing fetal bovine serum. Live measles virus was found to cause an almost complete inhibition of [³H]-thymidine incorporation in lymphocytes cultured in the presence of phytohemagglutinin, pokeweed mitogen, tuberculin purified protein derivate (PPD), or allogeneic lymphocytes. Analysis of cell size in the lymphocyte cultures revealed that blast transformation was inhibited as well. Measles virus, inactivated by heat or ultraviolet irradiation, did not cause inhibition. The inhibitory effect of measles virus was only measurable in the initial stages of culture; when added later, i.e., 24 hr before measuring [³H]-thymidine incorporation, it had no effect. The diminished reactivity of measles virus-infected lymphocytes cannot be explained by cytopathologic effects or by altered kinetics of lymphocyte transformation. When lymphocytes were cultured at 39 °C the extent of virus-induced suppression was significantly reduced. Very small amounts of pooled normal human serum, as well as IgG, prepared from the serum of a patient with subacute sclerosing panencephalitis, were able to prevent the inhibitory effect of measles virus.     

Rabbit antiserum to human B-cell alloantigens. II. Mechanism of inhibition of stimulation in the mixed lymphocyte response.

The process by which a rabbit antiserum to human B-cell alloantigens blocks stimulation in the mixed lymphocyte response (MLR) was investigated. A functional mammalian Fc region was necessary for the antiserum to be inhibitory, since F(ab′)₂ fragments failed to inhibit and a chicken antiserum with similar specificity to the rabbit anti-B-cell serum did not effectively block the response. Immune elimination of the stimulating cell population possibly via antibody dependent cell-mediated cytotoxicity (ADCC) or phagocytosis by macrophages was suggested by the observation that the addition of aggregated IgG to the MLR reduced the level of inhibition. It was also found that the number of immunoglobulin positive cells decreased in cultures treated with intact rabbit anti-B-cell serum, but not the corresponding F(ab′)₂ fragments, whether the cells were from a single individual or an allogeneic cell mixture. ADCC appears to be involved in the blocking process, as demonstrated by the marked reduction in MLR suppression when the MLR was initiated in the absence of ADCC effector cells. Removal or inhibition of monocytes in the MLR partially restored the response in experiments where the stimulator cells were pretreated with the antiserum, but not when the antiserum was present throughout the MLR.