Phenotypic expression of genetically controlled host resistance to Listeria monocytogenes

Protective effects of Chlorella vulgaris in lead-exposed mice infected with Listeria monocytogenes

Chlorella vulgaris extract (CVE) was examined for its chelating effects on the myelosuppression induced by lead in Listeria monocytogenes-infected mice. The reduction in the number of bone marrow granulocyte-macrophage progenitors (CFU-GM) observed after the infection was more severe in the groups previously exposed to lead. Extramedullar hematopoiesis, which was drastically increased after the infection, was not altered by the presence of lead. Treatment with CVE, given simultaneously or following lead exposure, restored to control values the myelosuppression observed in infected/lead-exposed mice and produced a significant increase in serum colony-stimulating activity. The benefits of the CVE treatment were also evident in the recovery of thymus weight, since the reduction produced by the infection was further potentiated by lead exposure. The efficacy of CVE was evident when infected and infected/lead-exposed mice were challenged with a lethal dose of L. monocytogenes after a 10-day treatment with 50 mg/kg CVE/day, given simultaneously to the exposure to 1300 ppm lead acetate in drinking water. Survival rates of 30% for the infected group and of 20% for the infected/lead-exposed groups were observed. Evidence that these protective effects of CVE are partly due to its chelating effect was given by the changes observed in blood lead levels. We have observed in the group receiving the CVE/lead simultaneous exposure a dramatic reduction of 66.03% in blood lead levels, when compared to lead-exposed nontreated control. On the other hand, CVE treatment following lead exposure produced a much less effective chelating effect. CVE treatments for 3 or 10 days, starting 24 h following lead exposure, produced a reduction in blood lead levels of 13.5% and 17%, respectively, compared to lead-exposed nontreated controls. The significantly better response observed with the simultaneous CVE/lead administration indicates that the immunomodulation effect of CVE plays an important role in the ability of this algae to reduce blood lead levels. In this regard, additional experiments with gene knockout C57BL/6 mice lacking a functional IFN-gamma gene demonstrated that this cytokine is of paramount importance in the protection afforded by CVE. The antibacterial evaluation measured by the rate of survival demonstrated that, in face of a 100% survival in the control group composed of normal C57BL/6 mice, which are resistant to L. monocytogenes, we observed no protection whatsoever in the IFN-gamma knockout C57BL/6 mice treated with CVE and inoculated with L. monocytogenes.

Immune alterations in three mouse strains following 2-deoxy-D-glucose administration

Using 2-deoxy-D-glucose (2-DG)-induced stress, our laboratory has developed studies to define stress effects on immune responses. Here, we report effects of increasing doses of 2-DG on the immune response of BALB/c, C57BL/6 and BDF(1) mice 2 h after three injections of 0 to 2000 mg/kg of 2-DG. Female 4- to 5-week-old mice were euthanized and blood and spleens were collected. A suspension of partially purified mature T splenocytes was obtained by negative selection using J11.d2 antibodies. Glucose and corticosterone levels were measured in the plasma of each mouse. Splenocyte and mature T splenocyte suspensions were tested in in vitro proliferation assays with or without concanavalin A. Splenocytes were analyzed for the following cell-surface markers: CD3, TCR alpha/beta, CD4, CD8 and major histocompatibility complex (MHC) Class II. Significant increases in blood glucose levels were observed in C57BL/6 and BALB/c strains with the highest 2-DG dose (p<0.05). Corticosterone levels were higher in BDF(1) mice and C57BL/6 mice following the administration of 1000 and 2000 mg/kg of 2-DG, respectively (p<0.01). In vitro proliferation of mature T splenocytes in the presence of concanavalin A was decreased in BDF(1) (p<0.05) but not in BALB/c and C57BL/6 mice. In addition, in BDF(1) mice the decrease was highly correlated with an increase of CD3+ and TCR alpha/beta+ cells in the spleen. These results demonstrated high variability in the response of different mouse strains to 2-DG-induced stress.

Studies in listeriosis show the strong symbiosis between the innate cellular system and the T-cell response

Resistance to infection with Listeria monocytogenes involves a series of cellular interactions, many of which are carried out by cytokines. Macrophages, NK cells and neutrophils participate in early stages of Listeria resistance. The neutrophil is specially important for clearance of the liver phase of listeriosis. Macrophages and NK cells interact by way of IL-12 and TNF, which induce the NK cell to produce IFN-gamma. IFN-gamma is the major macrophage-activating cytokine. The CB-17 SCID mouse shows these cellular interactions restricting the growth of Listeria, without its elimination. CD4 and/or CD8 T cells bring about sterilizing immunity. Macrophages influence the lymphocyte response by way of antigen presentation and also by promoting Th 1 differentiation. Thus, elimination of Listeria requires a symbiosis between innate immunity and the T-cell system.

Failure to demonstrate a major role for Kupffer cells and radiosensitive leukocytes in immunoglobulin-mediated elimination of Trypanosoma musculi

Previous studies have indicated that elimination of parasitemia in Trypanosoma musculi infection is brought about by immunoglobulin G2a antibodies, C3, and an effector cell. Experiments were designed to identify the putative effector cell by using several approaches. Infected C5-deficient or C5-sufficient mice treated with silica particles or given 900 rads of radiation 3 days earlier effectively eliminated trypanosomes following administration of immune plasma (IP). Silica-treated, noninfected mice given T. musculi preincubated with IP also cleared the parasites. Radiolabeling studies revealed that uptake of the cleared trypanosomes by the liver in normal mice was relatively low (24%) and fell only slightly (19%) in silica-treated mice. In contrast, uptake of radiolabeled sheep erythrocytes by the liver was normally much higher (47%) and fell drastically (7%) in silica-treated mice. Mice were then immunocompromised by 900 rads of radiation, silica particles, and anti-platelet serum combined before IP-sensitized trypanosomes were given. Leukocyte and platelet counts were both reduced by 95% and sheep erythrocyte uptake by the liver fell from 77 to 5%; however, greater than 99% of the injected trypanosomes were cleared in these mice and uptake of radiolabeled trypanosomes by the liver was similar to that of normal mice. Lastly, in anesthetized mice in which Kupffer cells were excluded surgically from the circulation, greater than 99% of the IP-sensitized trypanosomes disappeared rapidly from the blood. Only 7% of the radiolabel was found in the liver versus 60% in sham-operated mice. The results are interpreted as showing that hepatic Kupffer cells play a minor role in the immune elimination of T. musculi. Likewise, radiosensitive leukocytes and platelets are unlikely to be sole candidates for the putative effector cell that mediates a cure of murine trypanosomiasis.

Anti-Listeria monocytogenes immunity in mu-suppressed mice: a comparison of treatment with conventional hyperimmune rabbit anti-mouse IgM and affinity-purified, monoclonal rat anti-mouse IgM

The capacity of anti-IgM treated, B-cell-depleted mice to control infection by Listeria monocytogenes was evaluated. Suppression was achieved with a hyperimmune rabbit anti-mouse-IgM antiserum (IRS), with affinity-purified IRS (IRP), or with an affinity-purified, monoclonal, rat anti-mouse-IgM antibody (LO-MM-9). B-cell depletion in specifically treated mice was judged to be complete by the following criteria: absence of significant response to a B-cell mitogen lipopolysaccharide, absence of B-cells with detectable IgM or kappa light chain on their surface, absence of detectable IgM, and presence of free anti-IgM antibodies in serum. BALB/c mice, conventionally treated from birth with IRS, had an increased capacity to clear L. monocytogenes from the blood during the first 5 min after intravenous infection. Furthermore, control of infection seemed to be enhanced during the first 24 h but was found to be impaired when assessed 3 and 4 days after initiation of infection. These effects were, however, not IRS specific, because control mice treated with normal rabbit serum behaved comparably. Mortality caused by 2 x 10(3) L. monocytogenes injected intraperitoneally into BALB/c mice susceptible to L. monocytogenes was increased more in NRS-than in IRS-treated mice when both were compared with untreated control mice. Therefore, chronic injection of IRS or NRS seemed to disturb anti-L. monocytogenes immunity, rendering an evaluation of the role of antibodies impossible.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of responsiveness to the monocytosis-producing activity of Listeria monocytogenes in mice genetically susceptible or resistant to listeriosis

Injection of a monocytosis producing activity (MPA) from Listeria monocytogenes caused a dose-dependent elevation in numbers of macrophage colony forming units (CFU-m) in bone marrow and in peripheral blood. The increase could be seen as early as 8 h after injection of MPA and persisted until 72 h after injection. Serum from MPA-treated animals, capable of inducing monocytosis, also caused an elevation in numbers of CFU-m. The effect of MPA on CFU-m numbers in peripheral blood was measured in inbred strains of mice with differing susceptibility to listeriosis. Resistant C57B1/6 and B10. A mice responded best to MPA. C3H/HeJ and CBA mice, sensitive to Listeria, gave a slow, intermediate response. A/J mice, lacking the 5th component of complement, failed to respond to MPA. Since both B10.D2 old (C5-deficient) and B10.D2 new (C5-sufficient) mice, responded comparably to MPA, the unresponsiveness in A/J mice cannot be attributed solely to lack of C-5. Our results are consistent with the idea that A/J mice lack or are unresponsive to the MPA-induced serum factor which promotes monocytosis.

Experimental murine tularemia caused by Francisella tularensis, live vaccine strain: a model of acquired cellular resistance

We have established a model of experimentally-induced tularemia in mice, using the live vaccine strain of Francisella tularensis. A sublethal, intravenous inoculation of this organism caused in C57BL/6 strain mice an acute infection which lasted approximately 12 days. The clearance of Francisella from the bloodstream was shown to be complete by 5.5 hours postinfection. At this time, approximately twice as many bacteria were isolated from the spleen as from the liver. Mice which had recovered from a primary infection demonstrated a significant resistance to re-infection with autologous Francisella, a memory which persisted for at least 15 weeks. Resistance to experimental tularemia could be passively transferred from infected mice to naive mice by means of non-adherent spleen cells. Cells capable of adoptive transfer of resistance were present at a maximal concentration 7 days following infection, and persisted in significant numbers within the spleen cell population for at least 20 days after infection. Treatment of mice with serum from recovered animals caused a decrease in resistance when measured in the livers, and an increase in resistance when measured in the spleens. Suppression of T cell-mediated immunity during infection by treatment with cyclosporin A resulted in a dramatic increase in the tissue bacterial counts. Cyclosporin A-induced suppression of antitularemic resistance was first noted 2-3 days following infection and remained apparent for at least 8 days. The results of these experiments demonstrate that resistance to experimental murine tularemia is mediated predominantly by a cell-mediated mechanism. This mechanism involves T cells which become activated as early as 2-3 days following infection. Experimental, non-lethal infection with Francisella tularensis is thus an excellent model for investigating the mechanisms of acquired cellular immunity.

The relative difference in anti-Listeria resistance of C57BL/6 and A/J mice is not eliminated by active immunization or by transfer of Listeria-immune T cells

In this study, we examined the effects of active and adoptive immunization on the anti-Listeria resistance of innately resistant C57BL/6 and innately susceptible A/J mice. Although active immunization with a sublethal dose of viable Listeria monocytogenes markedly enhanced the anti-Listeria resistance of both C57BL/6 and A/J mice, the 100-fold difference between the two strains in innate anti-Listeria resistance was not diminished. Following immunization with an equivalent sublethal dose (0.1 LD50) of L. monocytogenes, both C57BL/6 and A/J mice generated T cells that could transfer significant and comparable protection to syngeneic recipients that were challenged with up to a 10 LD50 dose of L. monocytogenes. When the absolute number of viable Listeria was compared, however, it was clear that T cells from immunized C57BL/6 mice were capable of transferring protection to syngeneic recipients at Listeria challenge doses that were more than 100-fold greater than could T cells from Listeria-immunized A/J mice. Both active immunization and adoptive transfer of syngeneic Listeria-immune T cells enhanced the accumulation of inflammatory neutrophils and macrophages in C57BL/6 and A/J mice. More inflammatory neutrophils were recovered from actively immunized C57BL/6 than from A/J mice, whereas more inflammatory macrophages were obtained from adoptively immunized C57BL/6 than from A/J mice. These results provide further evidence for the beneficial role of inflammation in genetically determined innate resistance and T-cell mediated resistance to listeriosis. These data also suggest that some mechanism in addition to inflammatory responsiveness may be responsible for limiting the expression of acquired anti-Listeria resistance in genetically susceptible A/J mice.

The in vitro bactericidal activity of peritoneal and spleen cells from Listeria-resistant and -susceptible mouse strains

Two days after Listeria-resistant (LrR) C57BL/10 mice were infected intraperitoneally with Listeria, their peritoneal macrophages demonstrated enhanced bactericidal activity beyond that seen in susceptible (LrS) BALB/c or CBA mice. Intravenous infection had no effect on peritoneal cell activity. The induction, but not expression, of the enhanced activity was radiosensitive. There was no significant difference between the strains with respect to the number of cells or cellular composition of the exudates. No difference in the in vitro chemotactic response of cells from the two strains could be demonstrated. Therefore there seems to be recruitment to the infected peritoneal cavity of C57BL/10 mice of young, efficiently bactericidal monocytes/macrophages. On the other hand, spleen cell bactericidal activity was intrinsically superior in C57BL/10 mice compared with BALB/c mice, possibly because, as a haemopoietic organ, the C57BL/10 spleen already contains high numbers of these efficient monocytes.

Genetic control of murine listeriosis expressed in the macrophage response

Susceptibility to murine listeriosis is genetically regulated. For example, A/J, C3H, CBA, DBA/1, DBA/2 and 129/J mouse strains are classed as susceptible and demonstrate an early net bacterial growth rate which is significantly higher than that seen in strains classed as resistant, namely, C57BL-derived strains, NZB and SJL. These strain differences in susceptibility are expressed during the phase of natural resistance, as a property of the macrophage response. Genetic analysis in progeny derived from resistant C57BL-derived strains and susceptible A/J or BALB/c strain mice has indicated that a major gene is responsible for determining resistance/susceptibility to listeria. The genetic advantage of the resistant phenotype is attributed to a prompt influx of young (radiosensitive) inflammatory macrophages which control the early bacterial multiplication in infective sites. Such cells reportedly have superior listericidal activity in vitro, as compared to mature macrophages. Mononuclear phagocyte production, emigration and accumulation at infective foci are all increased in resistant C57BL, but not in susceptible A/J mice, shortly following infection. Thus, resistance to listeriosis is associated with an efficient macrophage inflammatory response and, conversely, susceptibility is attributed to a sluggish response. Genetic studies have demonstrated linkage between these two traits (listeria resistance/susceptibility and the macrophage inflammatory response). In all probability, different gene loci are responsible for susceptibility amongst the various mouse strains. In A/J mice, susceptibility is attributed to C5 deficiency (specified by Hc locus) while, for C5-sufficient strains, another genetic defect is presumably responsible.(ABSTRACT TRUNCATED AT 250 WORDS)

Listeria antigen-binding cells in the spleens of normal and immunized mice resistant or susceptible to listeriosis

Normal mice of the Listeria-resistant C57Bl/6 strain contain in their spleens a higher number of cells that bind Listeria monocytogenes cell wall fraction antigen (LmA) than normal DBA/2 mice, which are more susceptible to infection. LmA-binding cells are probably B cells, nylon-wool adherent, and inhibited by anti-mouse immunoglobulin antibody but not sensitive to the action of monoclonal anti-mouse macrophage and anti-Thy.1.2 antibody. A single intraperitoneal injection of 10(8) Listeria monocytogenes causes a rapid increase in the number of LmA-binding cells in the spleens of C57Bl/6 mice, and this can be seen as early as 24 h. On the other hand, in DBA/2 mice an increase in these cells becomes evident only by the 4th day. Moreover, the increment in the number of LmA-binding cells in C57Bl/6 mice is more marked than in DBA/2 mice.

Changes in serum colony-stimulating factor and monocytic progenitor cells during Listeria monocytogenes infection in mice

The capacity of a host to produce and mobilize monocytes is an essential component of host defenses during the early phases of infection caused by Listeria monocytogenes. In this study, the concentrations of colony stimulating factor (CSF) and the numbers of monocyte progenitor cells (CFUm) were measured in mice during infection with L. monocytogenes. The concentration of CSF in serum increased sharply during the first 24 h of infection and remained elevated for the next 7 days. The number of CFUm in the bone marrow, however, decreased during the first 4 days after injection of L. monocytogenes. Thereafter, the number increased slowly, returning to normal on day 14. The decrease in marrow progenitor cells did not appear to result from a reduced sensitivity to CSF. In contrast to bone marrow changes, spleen progenitor cells increased greater than 400%, reaching a peak 7 days after bacterial challenge. These data indicate that monocyte production during L. monocytogenes infection is correlated with a rise in serum CSF concentration, depletion of bone marrow CFUm, and an increase in the number of spleen CFUm.

Resistance to subcutaneous infection with Mycobacterium lepraemurium is controlled by more than one gene

The resistance of C57BL (high) and BALB/c (low) mice, their F1 hybrids, and the offspring derived from backcrosses of the F1 to both parental strains was assessed at 20 weeks after subcutaneous infection with 10(7) Mycobacterium lepraemurium organisms. The numbers of bacilli recovered from the infected foot and draining lymph node indicated that resistance to subcutaneous infection is controlled by more than one non-H-2-linked gene of intermediate dominance. In general, female mice were more resistant than males.

Effect of acute nutritional deprivation on immune function in mice. II. Response to sublethal radiation

Previous studies from this laboratory indicated that mice starved for 48 or 72 hr were resistant to the intracellular pathogen, Listeria monocytogenes. In the present experiments, we investigated the possibility that rapidly proliferating monocytes were responsible for the early protective effect observed in these mice. Confirming previous studies, the numbers of L. monocytogenes in livers and spleens of starved mice were 2-3 logs lower than those of fed mice 72 hr after inoculation of bacteria. The early protective effect of starvation could be eliminated completely by nonlethal doses of radiation (200-900 rads). Organ bacterial counts in starved-irradiated mice were similar to those of fed mice. Correlative histopathologic studies were carried out on all three groups of mice. Seventy-two hours after challenge with L. monocytogenes, the livers of fed mice had multiple microabscesses with cental necrosis and a poor mononuclear response. In contrast, livers of starved mice had fewer infectious foci, less necrosis, and a more prominent monocyte/macrophage inflammatory response. Similar to fed mice, the livers of starved-irradiated mice had marked necrosis and few monocytes/macrophages. In addition, the number of peripheral blood monocytes in starved mice was increased 72 hr after inoculation compared to fed and starved-irradiated mice. The data from these experiments suggest that a proliferating population of monocytes is responsible for resistance of starved mice against L. monocytogenes.

Murine encephalitozoonosis model for studying the host-parasite relationship of a chronic infection

Encephalitozoon cuniculi caused chronic nonlethal infections in euthymic BALB/cAnN mice, whereas athymic BALB/cAnN-nu mice died from infection. Specific, anamnestic, transferable, and acquired responses against E. cuniculi were expressed by infected euthymic mice. Resistance to lethal disease appears to be T-cell dependent. Immune serum failed to protect infected athymic mice, whereas the transfer of T-enriched spleen cells from E. cuniculi-sensitized euthymic donors prevented lethal E. cuniculi infections in athymic mice. These findings indicate that murine encephalitozoonosis may be an excellent system for studies of a chronic infection in an immunologically well-characterized host.

H-2 linkage control of resistance to subcutaneous infection with Mycobacterium lepraemurium

The H-2 linkage of the gene or genes controlling resistance to subcutaneous infection with 10(7) Mycobacterium lepraemurium organisms was investigated by using H-2 congenic strains on BALB and B10 backgrounds. Resistance was assessed by counting the organisms present at the infection site in the footpad and in the draining (right popliteal) lymph node 20 weeks after infection. When mice of BALB and B10 backgrounds with the same H-2 haplotype were compared, the BALB mice were always more susceptible. However, BALB/K (H-2k) mice were more susceptible than BALB/B (H-2b) mice, and BALB/B mice were more susceptible than BALB/c (H-2d) mice. There was no detectable difference in the resistance of B10.D2/n (H-2d) mice and B10 (H-2b) mice, but B10.BR (H-2k) mice were more susceptible than mice of the other two B10 strains. BALB/K was the only strain in which a high proportion of mice showed significant dissemination of organisms to the liver and spleen.

Role of macrophages in innate and acquired host resistance to experimental scrub typhus infection of inbred mice

Mechanisms of innate resistance to infection with the Gilliam strain of Rickettsia tsutsugamushi were examined using congenic strains of mice resistant (C3H/RV) or susceptible (C3H/He) to intraperitoneal infection. Both strains of mice were resistant to infection with 1,000 50% mouse lethal doses of rickettsiae if given intravenously. In both systems rickettsial replication occurred after intravenous infection, as evidenced by an increase in rickettsial numbers in the spleens of infected animals, followed by a decrease in rickettsiae to low levels by day 14 postinfection. Administration of the antimacrophage agents silica and carrageenan to C3H/He mice intravenously rendered these animals susceptible to lethal infection. Neither irradiation nor silica given individually rendered C3H/RV mice susceptible to intravenous infection. However, if silica and irradiation were given together, a lethal infection occurred after intravenous infection. C3H/RV mice became susceptible to lethal infection after sublethal doses of irradiation only if they were infected intraperitoneally. Administration of silica or carrageenan had no effect on the outcome of intraperitoneal infection of these mice with Gilliam rickettsiae. These data suggest that both strains of mice share innate resistance mechanisms to intravenous infection that consist of fixed macrophages. Resistance of C3H/RV mice to intraperitoneal infection, in contrast, apparently was dependent only on an irradiation-sensitive process.

Stimulation of monocyte production by an endogenous mediator induced by a component from Listeria monocytogenes

A monocytosis-producing activity (MPA) is present in a saline-extractable material (SE) from Listeria monocytogenes. The mechanism of stimulation of monocyte production by SE was studied. Serum obtained from mice at appropriate times after injection of SE induced monocytosis in normal recipients. The monocytosis-inducing activity present in serum differed from SE with respect to timing of the monocytosis, fractionation pattern on a Sephadex G-200 column, and thermolability. The minimum dose of SE capable of producing a monocytosis was 100 micrograms. Antibody to SE capable of detecting SE at a concentration of greater than 5 micrograms/ml failed to detect SE in samples of active serum. Therefore it seemed highly unlikely that activity in serum was due to the presence of trace amounts of SE. The activity present in serum was thermolabile and had a molecular weight of about 38,000. The data are consistent with the hypothesis that injection of SE caused the production or release of an endogenous mediator capable of stimulating monocytosis.

Measurement of 5'-nucleotidase in mouse peritoneal macrophages in listeriosis

5'-Nucleotidase activity in peritoneal macrophages of mice was measured both before and after infection with Listeria monocytogenes in hosts which possessed high or low anti-listerial resistance either due to a genetically determined trait or as a result of splenectomy. Reduction in enzyme activity was directly related to the degree of infection that developed in the hosts and hence was inversely related to the level of anti-listerial resistance observed. Basal 5'-nucleotidase activity was significantly lower in noninfected hosts that possessed high anti-listerial resistance, possibly reflecting a higher level of native macrophage activation in these hosts.

Cellular basis for genetically determined enhanced resistance of certain mouse strains to listeriosis

The characteristics of the mononuclear phagocytes mediating resistance to infection with Listeria during the early phase (0 to 48 h) of the response have been investigated in genetically determined susceptible (A/J) and resistant (C57BL/6, B10.A/SgSn) strains of mice. Irradiation immediately before infection profoundly enhanced the bacterial growth in the resistant strain, while having no effect in the susceptible strain, over a wide range (3 x 10(3) to 10(5)) of infective doses. This effect of irradiation is demonstrable at low-dose radiation (200 roentgens) and can be reversed by repopulation with 20 x 10(6) syngeneic nucleated bone marrow cells. Administration of dextran sulfate 500 24 h before infection profoundly enhanced the bacterial growth in the susceptible strain, while having much less effect in the resistant strain. Thus, the genetic advantage of the resistant mouse strains to listerial infection, at least during the early phase of the response, appears to be due to a cellular mechanism that is highly radiosensitive and relatively insensitive to dextran sulfate 500. In the susceptible strain, the early protective cellular mechanism is radioresistant and highly dextran sulfate 500 sensitive.

Splenic regulation of cell-mediated immunity to Listeria monocytogenes

Splenectomized mice are more resistant than normal mice to infection by Listeria monocytogenes. The nature of splenic regulation of cell mediated immunity to Listeria was investigated. Splenectomized mice were reconstituted with normal syngeneic spleen cells and normal plasma from Listeria-stimulated normal donors to determine if suppression of resistance in normal mice was cellular or humoral. Mice receiving spleen cells showed no decreased resistance, but mice receiving plasma showed decreased resistance as determined from bacterial numbers in the liver. The suppressive effect was associated with plasma components having a molecular weight less than 10,000. The data suggest that a suppressor factor is produced by spleen associated cells in response to stimulation of the cell-mediated immune system.