Accessory functions and mutual cooperation of murine macrophages and dendritic cells

IgG-mediated immunosuppression is not dependent on erythrocyte clearance or immunological evasion: implications for the mechanism of action of anti-D in the prevention of haemolytic disease of the newborn?

Haemolytic disease of the newborn (HDN) can be prevented by the passive administration of anti-D to the mother. The most accepted theory to describe this activity of anti-D is based upon its ability to clear opsonized erythrocytes before their recognition by the maternal immune system. We examined this hypothesis using a murine model of immunity to foreign erythrocytes. Whereas transfusion of foreign erythrocytes into mice induced immunoglobulin (Ig)M and IgG antibodies specific for the erythrocytes, these humoral immune responses were inhibited when the erythrocytes were opsonized with IgG. To specifically determine if immunological evasion occurs with these opsonized erythrocytes, we examined T-cell responses from these mice. An erythrocyte-specific T-cell response was clearly detected. We then tested whether phagocytosis of opsonized erythrocytes is sufficient to prevent the antibody response. We exposed mononuclear phagocytic cells to sheep red blood cells (SRBC) in vitro and then adoptively transferred the phagocytic cells to recipient mice; opsonized SRBC unexpectedly increased, rather than decreased, the antibody response. These data indicate that removal of opsonized erythrocytes by phagocytic cells does not prevent their immunological recognition and suggest that antigen clearance may not be the predominant mechanism of anti-erythrocyte action in downregulating the humoral immune response.

Differences in expression of cell surface co-stimulatory molecules, Toll-like receptor genes and secretion of IL-12 by bone marrow-derived dendritic cells from susceptible and resistant mouse strains in response to Coccidioides posadasii

Coccidioides posadasii is a soil fungus that causes coccidioidomycosis or Valley Fever in the endemic regions of the southwestern US and Central America. Persons with decreased T cells reactivity and immune deficiency are at increased risk of developing severe disseminated infection. Among different mouse strains, DBA/2 mice are relatively resistant to C. posadasii whereas BALB/c mice are highly susceptible, and this discrepancy has been attributed to the difference in the development and expression of their Th1 cellular response. Dendritic cells (DC) are the most potent antigen-presenting cells that are activated after taking up pathogens or pathogens-derived antigens and regulate the immune response in the host, including Th1 cellular response. However, the DC responses against C. posadasii are not characterized. In the present study, we cultured bone-marrow derived DC (BMDC) from BALB/c and DBA/2 mice and infected with C. posadasii arthroconidia. The activation of BMDC was characterized by studying expression of cell surface co-stimulatory molecules (CD11c, MHC class II, CD40, CD80, and CD86), expression of genes encoding Toll-like receptors and release of IL-12. We found that the BMDC from DBA/2 mice showed significant upregulation of Toll-like receptor-2 and 4 genes expression, secretion of IL-12 (p<0.05) and modest increase in T cell co-stimulatory molecules as compared to BMDC from BALB/c mice. The data suggest that the differences in the activation status of DC in DBA/2 and BALB/c mice may be responsible for the discrepancy in their susceptibility to C. posadasii.

Effect of prolactin on the antigen presenting function of monocyte-derived dendritic cells

Monocyte derived macrophages (Mphi) and dendritic cells (DC) play critical roles at the interface between innate and adaptive immunity. Both types of cells can effectively phagocytose exogenous antigens, whereas only DC can process and present them efficiently to antigen-specific T lymphocytes. The hormone PRL is also produced by immune cells and is regarded as a key component of the neuroendocrine--immune loop and a local regulator of lymphocyte response. Its main feature is cooperation with cytokines and hemopoietins. Triggering of monocyte PRL receptors with physiological-to-supraphysiological concentrations of PRL up-regulates the GM-CSF receptors, resulting in synergistic PRL-GM-CSF induced maturation of immature (i)DC. Further incubation induces increased antigen-presenting activity at the highest PRL concentrations studied (200 ng/ml). IFN-gamma, release by allogeneic lymphocytes is dependent on T cell-triggered IL-12 release by PRL-preincubated iDC. This, in turn, may be secondary to increased DC expression of CD40 or IFN-gamma. The permissive action of high PRL concentrations in the antigen presenting process may be of significance in initiation of the response against major histocompatibility complex (MHC)-presented self-antigens and may explain the association of hyperprolactinemia with autoimmune diseases.

Lung dendritic cells are primed by inhaled particulate antigens, and retain MHC class II/antigenic peptide complexes in hilar lymph nodes for a prolonged period of time

Intratracheal (IT) administration of heat-killed Listeria monocytogenes (HKL) results in an influx of macrophage and dendritic cell (DC) precursors into the lung interstitium. Low-density, FcR+, interstitial lung cells isolated from rats instilled 24 hr before with HKL or vehicle alone, were > 90% Mar1+. After culturing with granulocyte-macrophage colony-stimulating factor (GM-CSF) for 3 days, up to 24% of the loosely adherent cells were DC that stimulated allogeneic T-cell proliferation in an mixed lymphocyte reaction (MLR) assay. After only an overnight incubation with GM-CSF, however, the capacity of interstitial Mar1+ cells to stimulate HKL immune T-cell proliferation without exogenous antigen was low. By contrast, when DC were isolated as major histocompatibility complex (MHC) class II+ cells from rat lungs at 1, 3, 7 and 14 days after HKL instillation and cultured overnight with GM-CSF, their antigen presentation capacity without added exogenous antigen was robust, but declined over the 2-week period. Interestingly, hilar lymph node DC maintained their HKL antigen-presenting capacity for up to 2 weeks after instillation of HKL. Following IT administration of PKH-26 labelled HKL, fluorescent or immunolabelled organisms were detected in OX62+ DC in airway epithelium, lung interstitium and hilar lymph nodes in situ and in MHC class II+ DC isolated from these sites. We conclude that newly immigrated Mar1+ lung DC precursors, while efficient in endocytosing particulate antigens, are incapable of eliciting a significant proliferative response from HKL-sensitized T cells. By contrast, MHC class II+ DC isolated from lungs and incubated overnight with GM-CSF induce vigorous antigen-specific T-cell proliferation. Antigen-loaded lung DC in hilar lymph nodes maintain their antigen presentation capacity for up to 2 weeks.

CC chemokine receptor (CCR)2 is required for langerhans cell migration and localization of T helper cell type 1 (Th1)-inducing dendritic cells. Absence of CCR2 shifts the Leishmania major-resistant phenotype to a susceptible state dominated by Th2 cytokines, b cell outgrowth, and sustained neutrophilic inflammation

There is growing evidence that chemokines and their receptors regulate the movement and interaction of antigen-presenting cells such as dendritic cells (DCs) and T cells. We tested the hypothesis that the CC chemokine receptor (CCR)2 and CCR5 and the chemokine macrophage inflammatory protein (MIP)-1alpha, a ligand for CCR5, influence DC migration and localization. We found that deficiency of CCR2 but not CCR5 or MIP-1alpha led to distinct defects in DC biology. Langerhans cell (skin DC) density in CCR2-null mice was normal, and their ability to migrate into the dermis was intact; however, their migration to the draining lymph nodes was markedly impaired. CCR2-null mice had lower numbers of DCs in the spleen, and this was primarily due to a reduction in the CD8alpha(1) T helper cell type 1 (Th1)-inducing subset of DCs. Additionally, there was a block in the Leishmania major infection-induced relocalization of splenic DCs from the marginal zone to the T cell areas. We propose that these DC defects, in conjunction with increased expression of B lymphocyte chemoattractant, a B cell-specific chemokine, may collectively contribute to the striking B cell outgrowth and Th2 cytokine-biased nonhealing phenotype that we observed in CCR2-deficient mice infected with L. major. This disease phenotype in mice with an L. major-resistant genetic background but lacking CCR2 is strikingly reminiscent of that observed typically in mice with an L. major-susceptible genetic background. Thus, CCR2 is an important determinant of not only DC migration and localization but also the development of protective cell-mediated immune responses to L. major.

The beneficial effect of donor-specific transfusions: a review of existing explanations and a new hypothesis based on a relatively unapplied theory of T cell immunoregulation. A regulatory hypothesis in progress

The mechanism by which donor specific transfusions protect a graft from the recipient's immune system is unknown. It is likely that this beneficial mechanism is a subset or distinct exhibition of the general rules governing the regulation of the immune system. This phenomenon provides a strong framework for investigation of immune regulation, considering its potential consanguinity to immune regulation, that it is a paradox representing a manifestation of regulatory rules, and that it provides a wealth of clinical experience and experimentation from which to make inferences. Vital in any exploration of immune regulation, is the promise held in reducing the immune system to its chief elemental regulatory mechanisms and interactions. Strangely, the majority of this consequential work may have already been accomplished by Gershon, Green and colleagues with their elegant demarcation of T cell regulation into suppressor and contrasuppressor pathways. The practical and theoretical implications of this discovery seem to be, for the most part, ignored by mainstream immunology. It is doubtful, based on the quality and quantity of their work, or confirming work by other laboratories that they were inaccurate in their findings. It remains a horrible waste that their discoveries are not in immunology's pantheon of hallowed discoveries and are little used. With all this kept in mind, a comprehensive hypothesis of regulation was put together based mainly on Gershon's portrait of the suppressor and contrasuppressor pathways' contributions to immune regulation and experimentation surrounding the unsolved paradox of donor specific transfusions.

HLA-DR-mediated apoptosis susceptibility discriminates differentiation stages of dendritic/monocytic APC

Professional APC are characterized by their ability to present peptide via HLA class II in the presence of costimulatory molecules (CD40, CD80, and CD86). The efficiency of Ag presentation can be classed as follows: mature dendritic cells (DC) are most efficient, immature DC and macrophages are intermediate, and monocytes are considered poor APC. There is a large body of evidence demonstrating that HLA-DR transmits signals in the APC. In this study, we have addressed the question of the outcome of HLA-DR signals on APC of the monocyte/DC lineages throughout their differentiation from immature to mature APC. DC were generated from both monocytes and CD34+ cells of the same individual, macrophages were differentiated from monocytes. Immunophenotypical analysis clearly distinguished these populations. HLA-DR-mediated signals led to marked apoptosis in mature DC of either CD34 or monocytic origin. Significantly less apoptosis was observed in immature DC of either origin. Nonetheless, even immature DC were more susceptible to HLA-DR-mediated apoptosis than macrophages, whereas monocytes were resistant to HLA-DR-mediated apoptosis. The mechanism of HLA-DR-mediated apoptosis was independent of caspase activation. Taken together, these data lead to the notion that signals generated via HLA-DR lead to the demise of mature professional APC, thereby providing a means of limiting the immune response.

Cytokine production by rabbit alveolar macrophages: differences between activated and suppressor cell phenotypes

The differences between cytokine-producing profiles of activated macrophages (A-M phi) and suppressor macrophages (S-M phi) were examined. A-M phi, which exhibited cytotoxicity against RK-13 cells, were generated from resident rabbit alveolar M phi by treatment with lymphokine solution (culture fluids of rabbit spleen cells stimulated with concanavalin A [Con A]). S-M phi, which were able to inhibit cellular proliferations of rabbit spleen cells stimulated with Con A, were generated from resident alveolar M phi by treatment with 1-methyladenosine (an immunosuppressive molecule in tumourous ascites fluids). When A-M phi were stimulated with lipopolysaccharide (LPS) in vitro, the cells produced significantly more interleukin (IL)-1 (approximately 1.4 times), IL-6 (approximately 2.1 times), IL-12 (approximately 60 times), and tumour necrosis factor-alpha (TNF-alpha) (approximately 37 times) than did resting macrophages (R-M phi) stimulated with LPS as control cells. After the stimulation with LPS, both A-M phi and R-M phi did not produce transforming growth factor-beta (TGF-beta). In contrast, when S-M phi were stimulated with LPS in vitro, the cells produced significantly more TGF-beta (approximately 1.6 times) and significantly less IL-6 (approximately 1.8 times) than did control cells. Also, S-M phi did not produce IL-1, IL-12, and TNF-alpha into their culture fluids after the stimulation with LPS. These results show the differences between cytokine-producing profiles of A-M phi and S-M phi, and characteristics of their cytokine-producing profiles are analogous to T cell subsets. Differences displayed in the cytokine profiles may contribute to the effector (A-M phi) or the suppressor (S-M phi) functions of alveolar M phi.

Granulocyte-macrophage colony-stimulating factor overrides the immunosuppressive function of corticosteroids on rat pulmonary dendritic cells

Pulmonary dendritic cells (DC) are present in extremely small numbers, but they are the most potent antigen-presenting cells in the lungs. Pure populations of DC can be isolated from the lung following collagen digestion, Percoll gradient centrifugation, removal of phagocytic cells and flow cytometric sorting for cells which exhibit high levels of surface major histocompatibility complex (MHC) class II molecules. Exogenous GM-CSF enhances this immunostimulatory capacity of the pulmonary DC. Soluble factors produced by type II airway epithelial cells and interstitial macrophages also enhance the immunostimulating capacity of pulmonary DC while alveolar macrophages suppress it. Thus, the function of DC may be regulated by locally produced cytokines. Corticosteroids are widely used as immunosuppressive agents in pharmacotherapy. While these agents are known to inhibit T cell proliferation and macrophage activation, their effects on DC are not known. We found that dexamethasone (Dex) pretreatment resulted in about a 50% reduction in the immunostimulatory capacity of rat pulmonary DC. This was associated with downregulation of MHC class II (Ia) expression. Dex-induced suppression of DC function could be restored with GM-CSF. We conclude that corticosteroids downregulate antigen-presenting capacity by direct suppression of pulmonary DC. This immunosuppressive effect of corticosteroids on DC may, however, be abrogated by exogenous GM-CSF. Corticosteroids and GM-CSF are therapeutic agents with potent direct immunomodulating effects on DC.

Lymphoid dendritic accessory cells of the rat

The expression of MHC class II antigens on potential APC is a crucial step in T-lymphocyte activation and the initiation of an immune response. The studies which are presented here were initiated to characterize the critical APC present in physiologically normal, untreated rats. Such a cell should constitutively express these antigens at high density and therefore provide the apparatus necessary to provoke both primary and secondary immune responses at any time. DAC were found to fulfill these criteria. In the absence of specific surface markers of rat DAC, the results are based on the strict combination of morphological appearance and functional activity. However, the high expression of MHC class II antigens may be regarded as semispecific markers for DAC which are distributed at strategic positions in many lymphoid and nonlymphoid tissues (Hart & Fabre 1981, Steiniger et al. 1984). The relatively low number of these cells observed in tissue sections and in in vitro isolates (0.1% of all cells) may explain their high activity as APC. This would facilitate the presentation of antigen in vivo to a sufficient number of competent T lymphocytes. DAC differentiate from a bone marrow progenitor cell pool preferentially under the influence of spleen cell-derived activities. Although the exact lineage has not yet been determined it may be fair to speculate that DAC form a new cell lineage probably related to interdigitating cells but not to macrophages which differentiate from bone marrow-derived precursors under the influence of colony-stimulating activities. However, the cooperation between DAC plus macrophages may provide the stage for T-lymphocyte activation and T-T collaboration (Mitchison 1990). There are still many open questions concerning the general role of DAC in vivo and in vitro. To further characterize rat DAC, their tissue distribution, role in the immune response and possible influence on intrathymic lymphopoiesis, with respect to T-lymphocyte subpopulations and the selection of the T-lymphocyte antigen-receptor repertoire, a panel of DAC-specific monoclonal antibodies must be generated in the future. Such antibodies will also be useful to study the mechanism by which DAC activate T lymphocytes.

Collaboration between human blood dendritic cells and monocytes in antigen presentation

We compared human blood dendritic cells and monocytes for their capacity to produce secreted and membrane interleukin 1 (IL-1), stimulate mixed leukocyte reaction (MLR) and augment microbial antigen-induced T lymphocyte proliferation. Our enriched dendritic cell and monocyte fractions contained greater than 80% and greater than 93% dendritic cells and monocytes, respectively. Monocytes produced about ten times higher amounts of membrane and secreted IL-1 than dendritic cells, which in turn were more potent in presenting HLA-DR antigens in MLR. Both accessory cell types presented purified protein derivative of tuberculin (PPD) equally well, whereas monocytes were better with fixed Bacillus Calmette Guérin (BCG) bacteria. Processing of BCG was chloroquine-sensitive. Coculture experiments suggested that there was collaboration or synergy between dendritic cells and monocytes in antigen processing and presentation.

Enhancement of antigen-presenting function of dendritic cells with culture supernatants of mouse peritoneal macrophages stimulated with certain particulate substances

The production from murine resident peritoneal macrophages (M phi) of a soluble factor, which was capable of enhancing the antigen-presenting (AP) function of dendritic cells (DC), was examined. The supernatants of peritoneal M phi (M phi sup) were prepared by culturing peritoneal M phi with particles, i.e., zymosan A, latex, and sheep red blood cells (SRBC), or antigen-antibody (Ag-Ab) complexes such as keyhole limpet hemocyanin (KLH)-anti-KLH, ovalbumin (OVA)-anti-OVA, and SRBC-anti-SRBC complexes. When exposed to M phi sup during antigen pulsing DC induced a marked antigen-specific T cell proliferation, relative to DC treated with the supernatants from M phi cultured without stimuli (control sup). On the other hand, M phi sup-treated splenic M phi stimulated antigen-specific T cell activation to almost the same extent as did splenic M phi treated with control sup. These results indicated that peritoneal M phi elaborated a soluble factor which preferentially enhanced the AP capacity of DC when stimulated with particles or Ag-Ab complexes. Analytical gel filtration of M phi sup revealed that the factor had an apparent molecular weight of 27,000 daltons which was distinct from interleukin 1.

Dendritic cell activating factor produced by mouse macrophage hybridoma

A mouse macrophage (M phi) hybridoma which produces a soluble factor responsible for the cooperation between M phi and spleen dendritic cells (DC) was constructed. The antigen-presenting activity and the stimulator cell activity in the allogeneic or syngeneic mixed leukocyte reaction of DC were significantly augmented when DC were incubated with the culture supernatant of the hybridoma treated with various stimulants including latex beads. The monokine present in the culture supernatant of the hybridoma, called dendritic cell-activating factor (DCAF), augmented the production of lymphocyte-activating factor by DC while Ia expression of DCAF-treated DC was not altered. DCAF had no effect on the antigen-presenting activity of peritoneal resident M phi or B cell blasts while the antigen-presenting activity of spleen M phi was enhanced, but the degree of the enhancement was much less than that of spleen DC. DCAF was found to have the following properties: its pI value is between pH 4 and 5; it is stable at pH 2 to 10; and it loses its activity on incubation at 75 C for 30 min. When the culture supernatant of the hybridoma stimulated with latex beads was subjected to gel filtration, the DCAF activity was detected in the 20 Kd to 25 Kd, 30 Kd to 40 Kd, and 50 Kd to 60 Kd molecular weight regions. The 30 Kd to 40 Kd fraction, which is the major peak fraction, was further purified by ion-exchange chromatography followed by gel-filtration chromatography. When each fraction was subjected to SDS-PAGE, a 30 Kd band corresponding to the DCAF activity was observed and DCAF was purified to about 90% purity.

Functional deficiencies of spleen dendritic cells in autoimmune MRL/lpr mice

We investigated functional aspects of splenic dendritic cells (DC) obtained from MRL/MpJ-lpr/lpr (MRL/l) mice, and their normal counterparts, MRL/MpJ-+/+ (MRL/+) mice. In vitro antigen-presenting activity of DC obtained from MRL/l mice was impaired compared with that of DC from MRL/+ mice. Another major functional aspect of DC, i.e., stimulator cell activity in the allogeneic mixed lymphocyte reaction (MLR), was also found to be impaired in MRL/l mice. MRL/l mice and MRL/+ mice were also examined for co-operation between macrophages (M luminal diameter) and DC. We found peritoneal resident M luminal diameter obtained not only from MRL/+ mice but also from MRL/l mice released a soluble factor which enhanced the activity of DC from the respective mice. Furthermore, DC from both MRL/l and MRL/+ mice responded to the culture supernatant of a mouse macrophage hybridoma clone, S44, which releases a factor that enhances the antigen-presenting activity of DC, and their activity as stimulator cells in the allogeneic MLR was enhanced. However, the degree of enhancement was less in MRL/l mice than in MRL/+ mice.

Role of macrophages as modulators but not as autonomous accessory cells in the proliferative response of immune T cells to soluble antigen

The role of murine macrophages (M phi) and that of splenic dendritic cells (DC) were investigated in the antigen-specific proliferative response of memory T cells of mice primed with key-hole limpet hemocyanin (KLH) 6 weeks or more before. Peritoneal M phi, whether expressing Ia antigens or not, did not function as autonomous accessory cells (A cells). A-cell activity of the spleen adherent cell population, which comprised M phi in the majority and DC in the minority, was abolished by eliminating DC with a DC-specific monoclonal antibody and complement, and regained by the addition of a small number of DC. Though M phi did not function as autonomous A cells, they augmented the proliferative response in the presence of a small number of DC. This occurred not only in the presence of free antigen, but also when DC and/or M phi were pulsed with antigen. A culture supernatant of M phi having interleukin-1 activity was effective in enhancing the proliferation of T cells which responded to antigen-pulsed DC. On the other hand, interleukin-2 did not replace DC even in the presence of antigen-pulsed Ia+ M phi. We also investigated recently primed T cells, but no evidence was obtained in favor of the competence of M phi as autonomous A cells.

Interactions of Thermoactinomyces vulgaris and mouse alveolar macrophages. Loss of mitogenic activity and release of suppressive prostaglandins and interleukin 1

Interactions of mouse alveolar macrophages from three different inbred strains of mice and Thermoactinomyces vulgaris, a microbe associated with Farmer's lung disease, were studied. Alveolar macrophages were found to abolish the mitogenic activity of T. vulgaris. A prostaglandin synthesis inhibitor indomethacin, could not restore the activity. Alveolar macrophage supernatants generated by T. vulgaris treatment exerted strong suppression in secondary concanavalin A-induced lymphocyte transformation. Indomethacin partly relieved the suppression but a histamine 2 receptor blocker, cimetidine, had no effect. Interleukin 1 activity was practically undetectable by the thymocyte co-stimulation assay unless indomethacin was used. When indomethacin was used, interleukin 1 activity could be detected in all strains of mice tested. Major differences in the abolition of the mitogenic effect, in the suppressive effect, or in the release of interleukin 1 were not detected between inbred strains of mice tested. The results indicate that alveolar macrophages exert suppressive actions in vitro after T. vulgaris treatment but in vivo activities remain to be elucidated.

Analysis of the activation of lymphoid cells by mitogens in vitro with limiting dilution methods: adherent peritoneal cells suppress B-cell activation and synergize with WEHI-3 in the activation of T cells by Con A

Adherent peritoneal cells (APC) have often been used as a pure and effective macrophage population. Using partition analysis and small numbers of lymphoid cells activated by mitogens (concanavalin A for T cells (in the presence of TCGF) and LPS + DxS for B cells) we found that APC were accessory cells for T cell activation and growth but were not effective for B cells. Although APC were effective in assisting T-cell mitogenesis, they were not especially efficient. However, when APC were mixed with irradiated WEHI-3 cells (a tissue culture line previously shown to exhibit accessory cell activity in vitro for mitogenic activation T and B cells), the APC and WEHI-3 showed apparent synergy. One reason for failure of APC to assist B-cell mitogenesis was traced to the presence of a suppressor cell population which overcame the accessory cell help given by irradiated WEHI-3 cells to LPS-DxS stimulated murine B cells. It is thus possible to find "helper" effects (synergy of APC and WEHI-3 assisting the mitogenesis of T cells), as well as suppressor effects within the range of cells found in adherent accessory cells.

Antigen-presenting cell function of dendritic cells and macrophages in proliferative T cell responses to soluble and particulate antigens

The capacity of dendritic cells (DC) and macrophages (M phi) to present soluble and particulate antigen was tested in an ovalbumin (OVA)-specific T cell proliferation assay. In a comparative investigation we found that both DC and M phi were able to present soluble OVA, but that only M phi could present insolubilized OVA to T cells. DC were found to be able to present OVA in collaboration with M phi. The failure of DC to present insolubilized OVA is probably caused by their inability to endocytose these antigens. DC appeared not to endocytose substantial amounts of soluble OVA either. In contrast to M phi, antigen presentation by DC is not blocked by lysosomotropic drugs. Taken together, these observations suggest that DC can present soluble protein antigens without intracellular degradation.

Antigen presentation by spleen dendritic cells

It is now recognized that dendritic cells (DC) isolated from mouse spleen play an important role in activating T lymphocytes. These DC, which show many similarities to veiled cells found in the paracortical regions of spleen and lymph node, may be closely related to the epidermal Langerhans cells. It is known that DC are extremely effective allostimulators. We have also found that although DC lack demonstrable phagocytic ability, they are extremely potent at presenting soluble polypeptide antigens to primed T cells. Since T lymphocytes comprise several distinct subsets (particularly cytotoxic, helper, and suppressor) in our most recent studies we have asked whether DC are able to trigger all these different subsets of T cells. We examined the ability of different spleen cell types coupled with the hapten NP to induce antigen-specific T suppressors for a delayed type hypersensitivity (DTH) response. It was found that T suppressors were generated only when hapten was conjugated to a spleen-derived antigen-presenting cell. Further analysis revealed that macrophages but not DC were able to induce defined sets of suppressor cells in vivo (although DC were able to trigger a very powerful DTH response). We also examined the ability of DC to activate T cells which are required to cooperate with B cells in the production of antibody. Even though DC were able to trigger T lymphocytes to produce lymphokines, these activated T cells did not act as helper cells in a standard hapten-carrier system. Possible mechanisms for this dichotomy of DC function are discussed.

Hapten-specific T-cell response to azobenzenearsonate-N-acetyl-L-tyrosine in the Lewis rat. IV. Rat dendritic cells present soluble and insoluble azobenzenearsonate conjugates to T cells

We have used a fractionation procedure involving bovine serum albumin gradient floatation, adherence to glass, and rosetting with antibody-coated sheep erythrocytes to purify an accessory cell fraction from Lewis rat spleens. The fraction which is of light buoyant density, nonadherent, and FcR- is markedly Ia+, lacks phagocytic ability, is nonspecific esterase negative and under scanning electron microscopy has a heterogeneous morphology with a variety of protuberences described for rat dendritic cells. This putative dendritic cell preparation is very effective at stimulating proliferative responses with concanavalin A and allogeneic cells. When used to reconstitute the reactivity of peritoneal exudate cells of rats immunized with azobenzenearsonate-N-acetyl-L-tyrosine (ABA-Tyr) and subsequently depleted of Ia+ cells, it was shown to be highly effective with ABA conjugates of tyrosine, Ficoll, and polystyrene beads. Thus, despite the apparent absence of phagocytic or degradative abilities, this cell was very efficient at presenting large soluble and insoluble antigens. It is suggested that processing may occur at the cell surface without requiring internalization.

Dendritic cells and monocytes as accessory cells in T-cell responses in man. II. Function as antigen-presenting cells

The main antigen-presenting cells (APC) of human blood are reported to be the dendritic cells (DC), whereas monocytes (Mo) are only weakly or not at all capable of inducing T-cell immune responses to the soluble antigen purified protein derivative (PPD). In contrast, we found Mo to have a suppressive effect on the APC function of DC in vitro. Removal of Mo by adherence resulted in an increased APC activity, even though Mo produce more interleukin 1 (Il-1) than DC. Furthermore, addition of the prostaglandin synthesis inhibitor indomethacin gave rise to increased Il-1 production, HLA-class II expression, and a stronger antigen-specific T-cell response to PPD. Taken together, our studies indicate that the superior accessory cell function of DC compared with Mo in in vitro cultures may, at least partly, be attributed to the prostaglandin E2 production by Mo and more stable expression of HLA-class II molecules on DC.

Dendritic cells as accessory cells in antigen-specific murine T lymphocyte proliferation in vitro

The role of dendritic cells in antigen-induced murine T lymphocyte activation was studied by addition of purified dendritic cells to purified lymph node T lymphocytes from ovalbumin-primed mice. In the presence of the priming antigen T cells generated an antigen-specific response. The response was at least 3-fold higher with the use of a modified IMDM culture medium. The complete requirement for accessory cells was demonstrated only when nylon wool-purified T lymphocytes were thoroughly depleted of Ia antigen-expressing cells. Dendritic cells as well as peritoneal exudate macrophages were equally effective as antigen-presenting cells.

Mouse macrophage hybridomas secreting a cytotoxic factor and interleukin 1

Stable mouse macrophage hybridomas were produced by somatic cell fusion between proteose peptone-elicited peritoneal macrophages and NS-1 myeloma cells. Three cloned hybrid cell lines, designated as N/P-5-3, -6-2, and -7-1, exhibited typical macrophage-like morphology. Moreover, their macrophage characteristics were confirmed by the manifestation of intracellular nonspecific esterase, the detection of Mac-1 antigens and Fc-receptors on the cell surface, and the demonstration of phagocytic and antigen-presenting activities. Furthermore, these cell lines, stimulated with LPS, secreted considerable amounts of a cytotoxic factor and interleukin 1. Cultured cells of various tumor cell lines were sensitive to the cytotoxic factor, but normal thymocytes, spleen cells, and liver cells were not killed by the factor.

Macrophages as regulatory cells in mitogen induced spleen cell proliferation. 1. Macrophages as suppressor cells

In this study the effect of alveolar (AC) or peritoneal (PC) lavage cells from Lewis rats on the mitogen induced proliferation of syngeneic spleen cells was investigated. When total spleen cells were stimulated with PHA (5 micrograms/ml) or ConA (6 micrograms/ml), the addition of AC or PC and of their adherent (A) or non-adherent (NA) fractions had dose dependent suppressive effects. After passing the spleen cells through a Sephadex G-10 column, the eluted cells responded poorly to the mitogens. Addition of low numbers (2 X 10(3) = 1%) AC or NA-AC improved this response, whereas higher concentrations (5 or 10%) of these cells or any concentration of A-AC, PC, A-PC or NA-PC showed either none or a suppressive effect. The cells which strongly suppress mitogen induced lymphocyte proliferation have macrophage morphology, are adherent, esterase positive, bear Fc receptor for IgG and bind ganglioside coated erythrocytes. NA-AC were further separated into Ia-depleted (Ia-) and Ia-enriched (Ia+) population. The Ia+ fraction contained 80 to 90% Ia bearing cells, supported the mitogen induced stimulation of Sephadex G-10 depleted spleen cells and was ineffective in suppression. Interestingly, these cells did not bind ganglioside treated sheep erythrocytes. On the other hand, the Ia-negative NA-AC fraction and particularly the A-AC population strongly expressed the "ganglioside receptor" as well as typical macrophage markers like Fc receptors, latex-phagocytosis and non-specific esterase. Therefore, the "ganglioside receptor" may be a useful marker to define suppressor macrophages.

Heterogeneity of macrophages and dendritic cells as accessory cells

A number of different antigens was used to define the functional limits of Ia-bearing murine dendritic cells and macrophages in the processing and presentation of antigens for T cell activation. The results show considerable functional overlap as well as differences attributable to known properties of the cells. Thus both cell types could present soluble antigens up to the size of polymeric flagellin (M.W. in millions) about equally well. The nonphagocytic dendritic cells were most effective at inducing mixed leukocyte reactions in accordance with their high constitutive level of Ia expression. On the other hand, splenic macrophages were three to nine times better than dendritic cells at presenting particulate, heat-killed Corynebacterium parvum organisms to T cells lines, and small activated macrophages from bone marrow cultures were three times better again than splenic macrophages. Large activated bone marrow macrophages were not effective antigen presenters probably because of nonspecific suppression. These observations are consistent with the phagocytic and lysosomal activities of macrophages that enable them to ingest and process particulate antigen efficiently. Nevertheless, the capacity of dendritic cells and the dendritic-like line, P388.AD.4, to present particulate bacterial antigens suggests that these cells could either do the processing extracellularly or pick up soluble antigenic moieties shed from the bacteria and antigen processing macrophages. Glutaraldehyde fixation of C. parvum presumably stopped antigen shedding, since it produced a greater reduction of the T cell response with dendritic cells and P388.AD.4 as presenting cells than with macrophage presenters. Alternatively, the fixation could make the bacteria less "digestible" to dendritic cells than to macrophages. More characterization of the fate of antigens following encounter with accessory cells is necessary to distinguish between these possibilities.

Autostimulatory adherent cells in the spleen of aging mice: characterization in the syngeneic host-versus-graft reaction

Enlargement of the popliteal lymph node (PLN) of 6-week-old mice were elicited by the footpad injection of spleen cells of sex-matched, syngeneic, older mice, but not of 6-week-old mice, in a fashion of host-vs.-graft reaction. Effective stimulating cells in the inoculum seem to be Ia-bearing adherent cells. On the other hand, neither B and T cells nor immunoglobulin-secreting cells were effective for host T cell stimulation. Among nonlymphocytic adherent cells, only those attached to plastic dishes after a 24 h-incubation, enriched in macrophages, showed the stimulatory activity for the young recipients, while cells which had adhered once but detached and became non-adherent during a 24 h-incubation or a crude non-macrophage fraction did not induce the PLN response. Thus, the age-related antigenic change may occur on macrophages but not on dendritic cells. The effective cells should be alive, heat-killed cells being impotent to elicite the response. Such spleen adherent cells of aged mice were found to be also stimulative for age-matched recipients, when they are older than 3 months. Autostimulation by macrophages might be responsible, at least in part, for the age-related change of immune functions.