Production of macrophage migration inhibitory factor and lymphotoxin by leukocytes from normal and Wiskott-Aldrich syndrome patients

The production of macrophage migration inhibitory factor (MIF) and lymphotoxin (LT) by cultured leukocytes from patients with Wiskott-Aldrich syndrome (WAS) and normal controls was studied. The presence of these lymphokines in leukocyte culture supernatants usually correlated directly with the dose of stimulant used. Doses of nonspecific mitogens and specific antigens, which produced maximal in vitro lymphocyte transformation, stimulated maximal production of these mediators. When the incorporation of tritiated thymidine by stimulated leukocyte cultures from patients with Wiskott-Aldrich syndrome (WAS) was deficient, they usually produced less MIF and lymphotoxin than normal. However, when their in vitro lymphoproliferative responses were normal, the lymphotoxin activity in supernatants of WAS leukocyte cultures was normal.     

Macrophage migration from an agarose droplet: a micromethod for assay of delayed hypersensitivity in the mouse

The agarose droplet migration inhibitory factor (MIF) method was used to assay MIF synthesis by peritoneal exudate cells and spleen cells from mice immunized with M. tuberculosis H37Ra or EL-4 murine lymphoma. The results demonstrate the usefulness of this technically simple MIF method in the mouse, a species in which limited cell yields have complicated the use of other MIF methods for assessment of cell-mediated immunity.     

Host response to tissues placed in the anterior chamber of the eye: demonstration of migration inhibition factor and serum blocking activity.

Allograft immunity of rats to transplantation antigens was demonstrated by in vitro migration inhibition procedures. Spleen cell suspensions from rats sensitized to histocompatibility antigens by skin graft or anterior chamber implants failed to migrate normally in vitro when incubated in the presence of appropriate donor tissue extracts. Tissue grafts transplanted across both major and minor histocompatibility barriers had prolonged survival within the anterior chamber. However, 2 weeks after implantation, the recipient rats showed detectable sensitization to their implants. Serum from 14- to 32-day implant-bearing rats blocked the migration inhibition found in the implanted rats in the presence of corresponding tissue antigen. Such blocking activity was undetectable in the serum from rats which had implants for 7 days or after rejection of the implant was evident. MIF production in implanted and skin-grafted rats was evident at significantly different times in relation to the graft rejection. The asynchrony of MIF synthesis observed in these experimental animals leads us to postulate that the graft rejection and MIF production may be mediated by distinct lymphocyte populations. In addition, the route of the antigen presentation may account in part for the observed differences.     

Cellular immunity in man: correlation of leukocyte migration inhibition factor formation and delayed hypersensitivity

The formation of leukocyte migration inhibition factor (MIF) by the lymphocytes of 13 normal persons immune to the protein antigen keyhole limpet hemocyanin (KLH) has been investigated. KLH-induced MIF formation expressed as percent migration was compared with delayed hypersensitivity, antibody, and in vitro lymphocyte blastogenic responses to this antigen. Individuals were studied 404–840 days (median 540 days) after their last exposure to KLH. Nine persons had delayed hypersensitivity to KLH and 10 had circulating KLH antibody. The lymphocytes of 11 showed an in vitro blastogenic response to KLH stimulation, while the lymphocytes of nine produced MIF after KLH stimulation. The mean percent migration for the subjects with KLH delayed hypersensitivity was 48.2 (range 20.4–70.4) compared with 133 (range 120–161) for the four persons who did not have KLH delayed hypersensitivity (P < 0.05). The correlation coefficient between the precent migration and delayed hypersensitivity was ?0.78 (P < 0.01). No correlation was demonstrated between migration inhibition and the other parameters of immunity.     

Participation of three lymphoid cell types in the in vitro activation of cell-mediated immunity to a syngeneic gross virus-induced lymphoma in rats.

W/Fu rats inoculated with the syngeneic Gross-virus induced lymphoma, (C58NT)D, had transient lymphocyte-mediated specific cytotoxicity against the tumor cells at 7–15 days after tumor injection. Spleen cells 40 days after immunization (spleen 40) were unreactive by a direct 4-hr ⁵¹Cr release assay, but activity appeared after in vitro culture of these spleen eclls by themselves for 18–24 hr. The nature of the cells involved in the activation of cytotoxicity and the characteristics of the effector cells themselves were studied. Significant differences were seen in the cell types involved in the activation phase and the effector phase. Activation appeared to require the cooperation of three cell types. Induction of activity was lost by treatment of cells with ATS plus complement, by passage over an EAC-column, or by treatment with carbonyl iron. Thus, T cells, CRL and macrophages were necessary for full activation of cytotoxicity in spleen 40. In contrast, after activation, only CRL seemed to be required for cytotoxicity, and treatment wih ATS or carbonyl iron had little effect. The effector cell detected after in vitro activation was quite distinct from that seen in the direct cytotoxicity assay with spleen cells at 10 days after tumor cell inoculation. The early, direct cytotoxic reactivity was dependent on T cells, being eliminated by treatment with ATS and complement but not by EAC columns or carbonyl iron. It appears therefore that the in vitro activation is a separate mechanism for cytotoxicity against tumor cells, rather than a simple recovery of T cells from in vivo inhibition.     

Augmentation of antigen-specific cytotoxicity by concanavalin A at the peak of response to tumor allografts.

Antigen-specific cytotoxicity was augmented 24 hr after in vivo or in vitro treatment with concanavalin A at approximately the peak of response to allografted EL-4 cells. Such augmentation appears to require protein synthesis but not cell division. The augmentation may be ascribed to enhanced expression of cytotoxic activity of performed effector T cells rather than an increase in the number of effector cells.     

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.     

Enhancement by interferon of natural killer cell activity in mice.

Injection of mice with several interferon inducers, Newcastle Disease virus, polyinosinic-polycytidylic acid and tilorone resulted in an increase in spleen cell cytotoxicity for ⁵¹chromium-labeled mouse YAC tumor target cells in 4-hr in vitro assays. This increase in spleen cell cytotoxicity was abrogated by injection of mice with potent anti-mouse interferon globulin. Inoculation of mice with mouse interferon (but not human leucocyte or mock interferon preparations) also resulted in a marked enhancement of spleen cell cytotoxicity. The extent of enhancement of spleen cell cytotoxicity was directly proportional to the amount of interferon injected and a significant increase was observed after inoculation of as little as 10³ to 10⁴ units of interferon. An effect could be detected as soon as 1 hr after injection of interferon. The increase of spleen cell cytotoxicity after inoculation of an interferon inducer was not due to a localization and accumulation of cytotoxic cells in the spleen but reflected a general increase in cytotoxic cell activity in various lymphoid tissues (except the thymus). The splenic cytotoxic cells from interferon or interferon-inducer-injected mice had the characteristics of natural killer (NK) cells since (i) interferon enhanced spleen cell cytotoxicity in athymic (nu/nu) nude mice, (ii) classical spleen cell fractionation procedures by nylon wool columns, anti-Thy 1.2 serum plus complement, anti-Ig columns, and depletion of FcR+ rosette-forming cells, failed to remove the effector cells generated in vivo or in vitro. Therefore like NK cells, interferon-induced cytotoxic cells lack the surface markers of mature T and B lymphocytes, are not adherent, and are devoid of avid Fc receptors. Furthermore like NK cells, the spleen cells from interferon-treated mice lysed various target cells (known for their sensitivity to NK cells) without H-2 or species restriction. Incubation in vitro of normal spleen cells with interferon also resulted in an increase in cytotoxicity for YAC tumor cells. We conclude that interferon acts directly on NK cells and enhances the inherent cytotoxic activity of these cells.     

Metabolic and physiologic studies of nonimmune lymphoid cells cytotoxic for fibroblastic cells in vitro

An in vitro reaction between mouse lymphoid cells and target fibroblastic cells in wells of microtest plates, which appears to simulate the in vivo rejection of hemopoietic allografts, has been analyzed for metabolic and physiologic requirements. Protein synthesis was required for only the first few hours of culture. Inhibition of RNA synthesis and alteration of cell surface charge with various agents were without obvious effects. Metabolic slowing at 4 °C or deviation of the pH of the culture medium suppressed the reaction. Thymus cells, which are not cytotoxic in this system, significantly but not completely inhibited the cytotoxicity of lymph node cells. Antiserum directed against target cells specifically protected them from the cytotoxic lymphoid cells in the absence of complement.Precursors of cytotoxic lymphoid cells were radiosensitive, unlike the cytotoxic cells themselves. BALB/c anti-C57BL/6 spleen cell serum and ⁸⁹Sr both are able to prevent rejection of marrow allografts in vitro. Lymphoid cells incubated with this antiserum plus complement lost much of their cytotoxicity but were still effective at high ratios of aggressor to target cells. Lymphoid cells of mice treated with ⁸⁹Sr were effectively cytotoxic but lost practically all of their cytotoxicity afer incubation with the antiserum plus complement. Thus, it appears that this reaction detects two different cytotoxic lymphoid cells, either of which can function in vitro. Both cell types may need to cooperate in vivo during marrow allograft rejections.     

Contrasting effects of alloantiserum and xenoantiserum on cell-mediated lysis of tumor cells

The effect of anti-EL-4 serum on antibody-dependent cytotoxicity (ADCC) and cell-mediated cytotoxicity (CMC) was studied in allogeneic and xenogeneic systems. Inbred strains of BALB/c mice and Lewis rats were immunized with EL-4 tumor cells. Using microcytotoxic assays of ⁵¹Cr release from labeled EL-4 cells, complement-dependent cytolysis, ADCC, and CMC were determined. Complement-dependent cytolysis was observed in both systems. Although ADCC was demonstrated in both systems, the kinetics of cytolysis were different. Xenoantisera and alloantisera had opposite effects on CMC. Incubation of EL-4 target cells with BALB/c anti-EL-4 serum resulted in inhibition of CMC by immune BALB/c spleen cells. In contrast, treatment of EL-4 target cells with Lewis anti-EL-4 serum potentiated the CMC of immune Lewis spleen cells. It is thought that differences in the strength of response, antibody characteristics, and effector cells may determine the degree of inhibition or potentiation observed in these systems.     

Early antigen elimination by cytotoxic T cells results in diminished cellular immune responses to allogeneic tumor

Previous reports have suggested that repeated alloantigenic challenge increases humoral responses to alloantigens, but may cause decreasing cellular responses. We stimulated BALB/c (H-2^d) mice with intraperitoneal EL-4 tumor (H-2^b) and serially assessed cytotoxic responses in spleen and peritoneal lymphocytes using ⁵¹Cr-labeled EL-4 target cells. We observed that cytotoxicity generated in the spleen of nonimmune BALB/c mice was much greater than that in immunized mice; similar peak responses were generated in peritoneal lymphocytes in normal and immunized hosts. Complement-mediated cytotoxicity was not necessary for diminished splenic responses in hyperimmune hosts, for the same phenomenon was seen in the Hzl anti-BL/6 system which is free of humoral responses. Irradiation (2000 rad) of the EL-4 tumor challenge prevented tumor cell proliferation and markedly reduced splenic responses in nonimmune mice. We suggest that cytotoxic cells suppressed further generation of cyto-toxicity; by effecting an early elimination of tumor inoculum, tumor proliferation was abrogated and dose of cellular antigen was, consequently, markedly reduced.     

Thymus-dependent and thymus-independent effector functions of mouse lymphoid cells. Comparison of cytotoxicity and primary antibody formation in vitro

We have compared two effector functions, antibody formation and cytotoxic capacity in vitro, of mouse cells of various origin with special reference to the T lymphocyte dependence of these processes. We have used addition of PHA and coating of target chicken erythrocytes (CRBC) with antibody as the two means of inducing cytotoxicity. Antibody formation in vitro has been studied both against thymus-dependent sheep erythrocytes (SRBC) and thymus-independent (E. coli) antigens. Spleen cells from thymectomized, lethally irradiated bone marrow-, or fetal liver-repopulated mice were deprived of phagocytic cells by uptake of colloidal iron. They did perform better than normal spleen cells in the antibody-induced cytotoxicity and were also induced to cytotoxicity by PHA. PHA did not induce increased DNA synthesis in these T cell-deprived spleen cell preparations, which could not make primary antibodies to SRBC but were able to do so against E. coli antigens. Fresh bone marrow and fetal liver cells, deprived of phagocytic cells, were also induced into a highly efficient cytotoxicity by anti-CRBC as well as by PHA. Pretreatment of spleen cells with an alloantiserum (θ) against T lymphocytes reduced but did not abolish the PHA-induced cytotoxicity. In contrast, it did not affect the antibody-induced cytotoxicity. Such treated cells could not make antibodies to SRBC but could do so against E. coli. Pretreatment of spleen cells with a heteroantiserum (MBLA) against mouse B lymphocytes completely abolished all cytotoxic- and antibody-forming abilities of the cells, although experiments with combinations of θ-treated and MBLA-treated cells suggested that the MBLA treatment had left behind a significant portion of helper T cells needed for the in vitro antibody response. From these data we conclude, as have others, that the antibody-induced cytotoxicity is independent of T lymphocytes. It can be induced in immature precursor cells from fetal liver or bone marrow, and these cells may also become cytotoxic on interaction with PHA. However, in normal spleen cells, at least part of the PHA-induced cytotoxicity is T cell dependent. Some preliminary data suggest that this PHA-induced cytotoxicity of normal spleen cells may be a joint process between T lymphocytes and other cells.     

Involvement of lymphocyte mediators in the rejection of a murine tumor allograft

Macrophage migration inhibition by peritoneal leukocytes was studied in BALB/c mice bearing intraperitoneal allogeneic EL-4 lymphomas to explore the role of this immune effector function in allograft rejection. The nonadherent peritoneal leukocyte population harvested between 8 and 10 days after allograft inoculation inhibited migration of nonimmune murine macrophages as demonstrated by both direct and indirect migration assays using the agarose droplet method. This host response also contained large numbers of adherent macrophages which others have shown to be cytotoxic to EL-4 target cells. These findings provide direct evidence for lymphokine activity in allograft rejection and suggest that lymphocyte mediators may attract and activate the cytotoxic macrophages observed in this response.     

In vitro induction of primary and secondary xenoimmune responses by liposomes containing human colon tumor cell antigens

Liposomes prepared with human LS174T colon tumor cell membranes induce specific primary and secondary xenogeneic immune responses in BALB/c splenocytes in vitro. The multilamellar vesicular liposomes were prepared by adding sonicated membrane fragments in 8 mM CaCl₂ to a dried lipid film. Cytotoxic splenocytes generated in vivo exhibited specificity for the LS174T cell; liposomes elicited higher levels of cytotoxicity than did membranes (P < 0.01). Secondary blastogenic responses elicited in in vivo-primed spleen cells by liposome-antigens also produced a significantly greater (P < 0.005) response than membranes. Subsequently, in vitro induction of primary blastogenic and cytotoxic responses by liposome-antigens were accomplished and revealed similar kinetics to that of whole LS174T cell immunogens. Specificity of the in vitro-primed spleen cells was clearly demonstrated (P < 0.01) on a variety of human tumor cells using both the primed lymphocyte and cell-mediated cytotoxicity assays. The results of competitive inhibition tests with autologous lymphoblasts demonstrated that 30% of the cytotoxic activity was directed against lymphocyte antigens. Incorporation of tumor antigens into liposomes has thus enabled primary immunization in vitro to human colon cancer antigens and may afford an adaptable means to evaluate and to select desired immune responses, as well as to identify colon tumor-specific determinants.     

Cell-mediated immune response to syngeneic UV induced tumors: I. The presence of tumor associated macrophages and their possible role in the in Vitro generation of cytotoxic lymphocytes

A primary in vitro sensitization system employing a chromium release assay was utilized to investigate reactivity of murine spleen cells toward syngeneic ultraviolet (uv) light induced fibrosarcomas. These tumors are immunologically rejected in vivo when implanted into normal syngeneic mice but grow progressivly when implanted into syngeneic mice that had previously been irradiated with subcarcinogenic levels of uv light. Following appropriate sensitization, spleen cells from both normal and uv irradiated mice are capable of developing cytotoxic lymphocytes in vitro against the uv induced tumors. It was subsequently discovered that in situ uv induced tumors all contained macrophages of host origin that became demonstrable only after enzymatic dissociation of the tumor tissue. These macrophages were immunologically active in vitro as their presence in the stimulator cell population was necessary to achieve an optimum anti-tumor cytotoxic response following in vitro sensitization. Anti-tumor reactivity generated by mixing spleen cells and tumor cells in the absence of tumor derived macrophages could be greatly enhanced by the addition of normal syngeneic peritoneal macrophages. When in vitro anti-tumor reactivity of spleen cells from normal and uv treated mice was compared under these conditions we again found no significant difference in the magnitude of the responses. In addition, the cytotoxic cells generated in response to uv induced tumors appeared to be highly cross reactive with respect to their killing potential. Cross reactive killing was observed between all uv induced tumors tested as well as with a syngeneic benz[a]pyrene (BP) induced fibrosarcoma. No cytotoxicity was observed against normal syngeneic PEC's even through these cells were shown to be susceptible to lysis by anti-H-2^k effector cells. It was concluded that: (a) A significant number of host-derived macrophages are present in uv tumor tissue. (b) These macrophages are important for the in vitro generation of tumor specific cytotoxicity. (c) Spleen cells from uv treated mice are capable of recognizing and responding against uv tumor associated antigens in vitro. Cytotoxic effector cells generated in response to uv induced tumors appear to have specificity for tumor associated antigens (TAA) present on all uv tumors tested as well as a syngeneic BP induced tumor. The relationship between in vivo and in vitro reactivity against uv tumors is discussed.     

A nonspecific inhibitor of leukocyte migration in serum from rats bearing large fibrosarcomata.

A 3 M KCl crude extract of the syngeneic benzpyrene-induced fibrosarcoma termed BP 238 specifically inhibits migration out of glass capillary tubes of immune spleen cells from tumor amputee and small tumor-bearing rats, as does supernatant medium from tumor cells grown in culture. Serum from rats bearing small (< 2 cm³) tumors does not inhibit migration of immune spleen cells, while serum from rats with larger tumors (>4 cm³) nonspecifically inhibits migration of both immune and nonimmune spleen cells, thoracic duct lymphocytes, and thymocytes. This nonspecific inhibition increases with increasing tumor size, does not correlate with the presence of bacterial infection, and is presumably due to a circulating factor produced in vivo during tumor growth. Production of macrophage inhibitory-like factor (MILF) by neoplasms in vivo may offer a mechanism for tumors to escape immunosurveillance by systemic immobilization of cytotoxic lymphocytes. From Sephadex and ultrafiltration fractionation experiments, the molecular weight of MILF in serum is polydisperse (30,000–100,000 daltons), and is heat and chymotrypsin resistant, in contrast with the properties reported for LIF (leukocyte inhibitory factor) and MIF (macrophage migration inhibitory factor) produced in vitro.     

In vitro stimulation of mouse NK cell activity by phorbol esters: A pathway different from interferon-induced activation

Tumor promoters of the phorbol diester type were tested in vitro for their effect on mouse natural killer (NK) activity. The 12-O-tetradecanoyl-phorbol-13-acetate (TPA) significantly increased the spontaneous cytotoxicity of spleen cells preincubated with this reagent, with a maximal effect at 10–20 ng/ml. Higher concentrations of the weak tumor promoter phorbol-12, 13-dibenzoate (PDB) were required to display the same NK-enhancing property, while the inactive tumor promoter 4-αphorboldidecanoate was ineffective at any concentration. TPA-responding spleen cells displayed the same characteristics as classical NK cells: they were present in nude mice, did not adhere to nylon wool, bore the Thy-1 antigen, and displayed the same target cell specificity as interferon-activated NK cells. A major difference with the interferon-induced NK activation resides in the fact that the TPA-inducible increase in lytic activity does not require RNA and protein synthesis. Our results suggest two different NK activation pathways for IFN and phorbol esters.     

Suppressor factor from tumor-allosensitized spleen cells--its effect on in vitro proliferation of tumor cells and in vivo skin allograft survival

C57BL/6 mice are sensitized ip with allogeneic P-815 mastocytoma cells. Fifteen days later the spleen cells of the sensitized mice are used in the production of suppressor factor or treated with mitomycin and used as suppressor cells. Sensitized spleen cells incubated with the specific alloantigen (DBA/2 m-treated spleen cells) release suppressor factor (SF)² which inhibits cell proliferation in mixed lymphocyte culture (MLC) as well as the in vitro generation of cytotoxic cells (CML). SF is most effective when added eary during MLC. SF also inhibits mitogen responsiveness of normal spleen cells. In addition to inhibiting lymphocyte function in vitro, suppressor cells as well as SF inhibit the in vitro proliferation of tumor cells. This inhibition is specific for the tumor to which the suppressor cells are induced. The inhibition of tumor cell proliferation is not due to the presence of cytotoxic cells in the spleen of the tumor-allosensitized mice. Suppressor cells from neonatal mice do not inhibit the in vitro proliferation of tumor cells. SF injected iv into C57BL/6 mice decreases the mixed lymphocyte reactivity of the host spleen cells and decreases the ability of the host to reject skin allografts. We interpret these data to suggest that tumor-allosensitized spleen cells, and the SF they produce, not only affect lymphocyte function but also inhibit tumor cell proliferation. This dual effect of suppressor cells could be an important part of the immune surveillance against tumors.     

Murine T-cell hybridomas that produce lymphokine with macrophage-activating factor activity as a constitutive product

Responder spleen cells primed to alloantigens in vivo could generate high degree of cytotoxicity against low- or nonimmunogenic stimulators such as thymocytes or uv light-treated spleen cells in vitro. However, a removal of adherent cells from primed responder cells remarkably reduced the cytotoxicity after stimulation with such low-immunogenic stimulators. Adding a small number of peritoneal adherent cells (PACs) also suppressed the cytotoxic activity of unseparated responders against low-immunogenic stimulators. These suppressive effects by PACs were blocked by indomethacin. By adding prostaglandin E₂, cytotoxic T lymphocyte (CTL) generation of primed unseparated responders against low-immunogenic stimulators was suppressed; however, cytotoxic activity against mitomycin C-treated stimulators was not suppressed. These results suggested that prostaglandins released from PACs selectively inhibited the function of splenic adherent cells that were required for CTL generation of primed responder spleen cells against low-immunogenic stimulators in vitro.     

Macrophages as effector cells of protective immunity in murine schistosomiasis: IV. Coincident induction of macrophage activation for extracellular killing of schistosomula and tumor cells

Peritoneal macrophages from Schistosoma mansoni-infected mice are activated both for nonspecific tumor cytotoxicity and for killing of skin-stage schistosomula in vitro. In the current study, mechanisms for induction of macrophage tumoricidal and schistosomulacidal activity have been compared. Examination of macrophages activated in vivo by BCG infection or C. parvum treatment, or in vitro by exposure to lymphokine prepared from antigen-stimulated BCG-immune spleen cells, showed that these effector functions were closely linked. Indeed, fractionation of lymphokine-rich supernatant fluids by Sephadex G-100 gel filtraction showed that activities responsible for induction of schistomula killing by inflammatory macrophages and for induction of tumoricidal activity cochromatographed as a single peak in the 50,000 MW region. Thus, development of macrophage-mediated cytotoxicity against these two extracellular (tumor cell or helminth) targets was coincident in several cell populations activated in vivo or in vitro. However, activation for tumoricidal and schistosomulacidal capacity appeared to be quantitatively dissociated in macrophages from mice with chronic schistosomiasis; those cells demonstrated low, yet significant, levels of larval killing ($\text{1}\text{3}$ to $\text{1}\text{5}$ those of BCG or lymphokine-activated cells) but maximal levels of tumor cell cytotoxicity. Furthermore, cytotoxicity by peritoneal cells from S. mansoni-infected mice was not increased in vitro by exposure to lymphokine. Identification of this functional alteration in S. mansoni-activated cells may help to clarify the role of macrophages in the partial immunity against challenge infection which is demonstrated by mice with chronic primary S. mansoni infection.