The peritoneal antigen-presenting macrophage: control and immunogenic properties of distinct subpopulations

Fas expression on peritoneal macrophages during continuous ambulatory peritoneal dialysis peritonitis

BACKGROUND

Peritonitis is a common complication of end stage renal failure (ESRF) patients receiving continuous ambulatory peritoneal dialysis (CAPD). Peritoneal macrophage may participate in the activation of specific T cells and in the generation of local cell-mediated immunity to various pathogens. The purpose of this study is to investigate the possible role of macrophage in CAPD patients with peritonitis.

METHODS

We evaluated the expression of Fas receptor (CD95), ICAM-1 (CD54), CD25, and CD69 by two-color flow cytometry on extravasted macrophages from 16 ESRF patients on CAPD with peritonitis (peritonitis-positive) and compared them to 11 ESRF patients on CAPD without peritonitis (peritonitis-negative) and normal controls.

RESULTS

We found an increased expression of CD95, CD54, and CD25 on macrophage in peritonitis-positive group compared to controls (all p < 0.001). In the peritonitis-positive group, the CD95 expression was significantly higher than that of the peritonitis-negative group (p < 0.001). The expression of CD54, CD25, and CD69, however, was not significantly different between the peritonitis-positive and peritonitis-negative CAPD subgroups.

CONCLUSION

We found an abnormally increased percentage of macrophage-expressing Fas receptor and ICAM-1, and the percentage of CD95+ macrophage, but not those of other markers, were increased among the subset of CAPD patients with peritonitis. The later finding suggests that this macrophage phenotype is associated with peritonitis occurring in CAPD.

Subpopulations of mouse resident peritoneal macrophages fractionated on Percoll gradients show differences in cell size, lectin binding and antigen expression suggestive of different stages of maturation

Aiming the identification of macrophage heterogeneity, mouse resident peritoneal cells were fractionated on discontinuous Percoll gradients into six discrete fractions (0, 1, 2, 3, 4 and 5 in order of increasing density). All six fractions and the total population were characterized by light and electron microscopy, and flow cytometry. The least dense fraction (0) had a low viability (44%); fractions 4 and 5 had a low percentage of macrophages. Light microscopy and flow cytometry of macrophage-enriched fractions 1, 2 and 3 showed an inverse correlation between diameter and cell density, as well as suitable differences in lectin binding to the macrophages of each fraction. The surface of macrophages from fraction 1 had more sialyl residues (higher binding of the lectin LFA), less galactosyl residues (lower binding of the lectin PNA) and expressed more receptors for the antibodies M1/70 (Mac-1) and F4/80; fraction 3 had an opposite binding behavior for the lectins and expressed fewer receptors for both antibodies; fraction 2 had an intermediate behavior for both parameters. Binding of the lectins Con A and HPA showed slight differences, whereas UEA I did not present a detectable difference among the fractions analyzed. These findings suggest that the macrophage heterogeneity achieved by the gradient separation of resident peritoneal cells could be explained by different stages of macrophage maturation.

Antigen-presenting function of human peritoneum mesothelial cells

Mesothelial cells (MC) from human peritoneal omentum fragments obtained during surgical insertion of peritoneal catheters for continuous peritoneal dialysis in end stage renal failure (ESRF) patients were cultured in vitro. MC exhibited a phenotype different from macrophages, but MHC class II molecules were well expressed. Therefore MC lines were tested for antigen-presenting capacity by pulsing with soluble antigens (tetanus toxoid and purified protein derivative (PPD)) or with a corpusculate antigen (Candida albicans bodies). Autologous peripheral blood mononuclear cells (PBMC) depleted of adherent monocytes and cloned T cells generated from an individual matched for the MHC class II antigen DR2 were used to test antigen-presenting function. MC effectively presented the soluble and corpusculate antigens to autologous and MHC-compatible allogeneic lymphocytes, indicating that they are endowed with both endocytic/phagocytic activity and with processing/presenting capacity. Preincubation of MC with human recombinant interferon-gamma (IFN-gamma) up-regulated MHC class II and intercellular adhesion molecule-1 (ICAM-1) expression, but the effect on antigen-presenting function was not consistent. Since MC are an important component of the peritoneal environment, they may participate, along with macrophages, in activation of specific T cells and in the generation of local cell-mediated immunity to various pathogens.

Separation of murine peritoneal macrophages using Percoll density gradients

Macrophages harvested from the murine peritoneal cavity are functionally and morphologically heterogeneous. Here, we describe a procedure which permits the determination of specific cell densities using a continuous density gradient of Percoll (analytical step). Subsequently, discontinuous density gradients are used in routine (preparative step) to isolate all the cell subpopulations according to their actual specific density. This procedure has been successfully used for both mouse and rat peritoneal macrophages.

Quantification of bacterial phagocytosis by flow cytometry and spectrofluorimetry

Bacterial phagocytosis is a cardinal function of phagocytes. We describe a simple procedure to easily quantify this function using fluoresceinated bacteria. Non-ingested bacteria and those adsorbed to the cell membrane are eliminated by an enzymatic procedure. Only macrophages with ingested fluorescent bacteria are detected, thereby permitting an accurate quantification of the phagocytic process by both spectrofluorimetric measurement and flow cytometric analysis.

Effect of ambient temperature on phenotype and functions of professional phagocytes of athymic nude mice

Cytofluorometric analysis of surface marker expression was performed on myeloid cells isolated from bone marrow, spleen and lymph nodes of nude mice and nu/+ and +/+ mice (haired controls) exposed for various time periods to ambient temperature of 22 degrees C or 28 degrees C. A rise in the proportion of cells bearing macrophage markers (MAC-1, MAC-3 and F4/80) in the spleen and of FcR+ cells in all tissues tested was found in 22 degrees C-exposed nudes with high nonshivering thermogenesis. Numbers of MAC-1+ macrophages and actively phagocytic cells increased also in peritoneal exudates. There was a conspicuous predominance of large macrophages in the exudates and the specific markers decreased in density on their surface. Ia expression declined in all tissues tested with the length of exposure to cold. In the granulocytic series (BP-2+ cells), there was a decrease in the bone marrow and lymph nodes and an increase in the spleen and circulation, which suggested an enhanced mobilization and increased production at extramedullary sites in cold-exposed nude mice. The changes in haired mice were negligible.

Mouse peritoneal macrophages: characterization of functional subsets following Percoll density gradients

Mouse resident peritoneal cells were separated into twelve fractions on Percoll gradients according to their specific density and were thoroughly characterized by Giemsa staining, some biochemical assays, immunophenotyping and phagocytic tests. Among these fractions, the macrophages were mainly represented in 7 subsets of 1.073 to 1.104 g/ml densities. The results of this study emphasize that resident peritoneal macrophages of primo-explantation can be divided into two distinct subpopulations with separate functions, related to the stage of cell maturity. In fact, our results show that one macrophage subpopulation is rich in immature cells, characterized by their peroxidative activity, the expression of F4/80 antigen, Mac-1 and Fc receptors, in correlation with their high specific density; the second subpopulation contains mature macrophages (lower percentage of peroxidase-positive cells) with lower densities and a lower level of expression of the above-mentioned molecules. Antibody-dependent and antibody-independent bacterial phagocytosis, the phagocytic index and Fc gamma RII rosetting increased together with the cell density, and were elevated in the immature cell subpopulation. T and B lymphocytes were also identified in all the macrophage subsets, but in a low proportion.

In vitro cytokine release by activated murine peritoneal macrophages: role of prostaglandins in the differential regulation of tumor necrosis factor alpha, interleukin 1, and interleukin 6

Distinct subpopulations of macrophages or differently activated macrophages display various functions in immune reactions. Some of their activities depend on specific sets of factors (i.e., cytokines and eicosanoids) produced by activated macrophages. We have studied the ability of murine (CBA/ca) peritoneal macrophages to selectively release tumor necrosis factor alpha (TNF-alpha), interleukin 1 (IL-1), and IL-6. We have found that the priming of cells (Mo) with different stimulants (thioglycolate vs. LPS) induces the release of particular cytokines by reactivated macrophages. The increased release of TNF-alpha correlates with lower levels of IL-1 and IL-6. We have also found that prostaglandin E2 (PGE2) and prostacyclin (PGI2) have opposing effects on the production of two of these cytokines. The release of TNF-alpha is inhibited by prostaglandins, whereas increased levels of PGE2 and PGI2 correlate with higher levels of IL-6.

Changes in macrophage populations: phenotypic differences between normal and tumor-bearing host macrophages

As a tumor grows, changes occur in the function of macrophages (M phi). This is concomitant with changes in their phenotype. Flow cytometric analysis of monoclonal antibody (mAb)-labeled thioglycollate-elicited peritoneal, and resident splenic, M phi showed a tumor-induced shift of Mac-1, -2, -3, and Ia antigen expression. During tumor growth, the percentage of peritoneal Mac-2+, -3+, and Ia+ M phi decreased significantly (22%, 14%, and 58%, respectively), while Mac-1+ M phi remained unchanged. By analyzing the data on two-dimensional histograms and comparing the sizes of M phi to cell-surface antigen expression, we identified distinct subpopulations of peritoneal M phi. Three distinct size versus antigen expression M phi subpopulations were detected by flow cytometry and consisted of 10-16, 17-22, and 23-27 microns for the small-, medium-, and large-sized populations, respectively. Large-sized Mac-1+ and -2+ M phi decreased (37% and 38%), while large-sized Mac-3+ M phi did not decrease during tumor growth. Medium-sized Mac-3+ M phi decreased 33% during tumor growth, while no differences could be seen in medium-sized Mac-1+ or -2+ M phi. Concomitant with the decrease in large-sized Mac-1+ M phi was an increase in small-sized Mac-1+ M phi. Peritoneal Ia+ M phi were mostly small-sized (4-7-fold increase over the medium-sized and none in the large-sized population). M phi Ia antigen expression was nearly absent in the 21-day tumor-bearing host, with less than 4% of the cells labeling positive (a 73% drop from normal host M phi). In splenic M phi, the percentage of Mac-1+ M phi significantly increased (90%) during tumor growth, while Mac-2+ and -3+ M phi showed a smaller, but still significant, increase (48% and 40%, respectively). Additionally, splenic Ia+ M phi significantly decreased (29%) during tumor growth. More important than the decreased cell numbers was the significant decrease in Ia antigen expression per cell. Unlike the peritoneal M phi, the splenic M phi did not show distinct size versus antigen expression subpopulations, although there was an overall difference in M phi size between normal and TBH. These data suggested that M phi from different anatomical sites are phenotypically different and tumor growth mediates phenotypic alterations in peritoneal and splenic M phi populations. This may be the source of tumor-induced dysfunction of M phi-mediated immune activity.

Immunogenic capacity of macrophage hybridomas

Two clones, E2-7.7 and E2-10.50, derived from two macrophage(M phi)hybridomas, E2-7 and E2-10, have been studied. The first clone, E2-7.7, is Ia+ and Fc receptor (FcR) negative and manifests a strong antigen-presenting capacity. When we pulsed its cells in vitro with keyhole limpet hemocyanin (KLH) antigen and injected them into syngeneic animals, we found that as small a dose as 10(3) cells initiated an immune response in vivo. On the other hand, antigen-pulsed cells of the E2-10.50 clone, which are Ia- and FcR+, were almost incapable of triggering immunity, even when injected at a dose of 10(5) cells. Thus, the two clones differ in their immunogenic capacity (both cellular and humoral immunity). In experiments aimed at testing the stimulation in vitro of primed lymph node (LN) cells by antigen-pulsed cells of these two hybridoma clones, we observed that E2-7.7 stimulated the unfractionated population of LN cells and the LN-derived population of T cells. The E2-10.50 cells stimulated only the unfractionated population of LN cells, but not the T cell population. Subsequent tests indicated that the E2-10.50 cells require an intermediate Ia+ accessory cell to present the antigen to the T lymphocytes. Analyzing the molecular structure of the M phi hybridomas, we discovered that major histocompatibility complex (MHC) genes of the myeloma haplotype (H-2d), and of the splenic M phi used for fusion (H-2k), which were not expressed in the parental myeloma or in the E2-10.50, were expressed in the E2-7.7. Thus, somatic cell fusion of M phi resulted in the activation of suppressed genes of the myeloma partner. It appears that these antigens participate in controlling the immunogenic properties of the E2-7.7 clone. Testing the effects of interferons on the M phi hybridomas, we observed that interferon-gamma activated, at both the mRNA and the cell surface-antigen levels, the expression of H-2Dk, H-2Kd and H-2Dd in the E2-10.50 cells, but not in the E2-7.7. Consequently, interferon-gamma augmented significantly antigen presentation by E2-10.50 but not by E2-7.7 cells. These two hybridoma clones might represent two distinct subsets of normal M phi, manifesting two different sets of functional properties.(ABSTRACT TRUNCATED AT 400 WORDS)

In vitro studies of the role of monocytes in the immunosuppression associated with natural measles virus infections

Measles is associated with suppression of mitogen-induced proliferative responses. The role of monocytes in depressed responses to phytohemagglutinin during measles was assessed. Depleting peripheral blood mononuclear cells of adherent cells decreased, increased, and did not change proliferative responses in essentially equal numbers of individuals. Inhibition of prostaglandin synthesis by indomethacin significantly increased proliferation (P = 0.009) but often not to normal values. Addition of supplemental interleukin 1 had little effect. Addition of supplemental natural interleukin 2 significantly increased proliferation (P = 0.002) even in patients with very low responses. These data suggest that monocyte function is abnormal in some individuals with measles, that the abnormality is variable, and that lymphocyte response to monocyte signals is probably suboptimal as well.

Induction of a macrophage-suppressive lymphokine by soluble cryptococcal antigens and its association with models of immunologic tolerance

Soluble extracts of Cryptococcus neoformans were examined for their ability to induce a macrophage-regulatory T-suppressor cell known to appear in the spleens of mice infected with cryptococci. Suppressor cells were induced by injection of extracts of encapsulated or thinly encapsulated strains of cryptococci. Dose-response analysis showed that as little as 25 micrograms of soluble capsular polysaccharide antigen could induce significant suppressor cell activity, with maximum suppression occurring at a dose of 100 micrograms. The suppressor cells appeared within 1 week of injection of antigen and persisted for at least 2 months. Suppressor cells were induced in animals given tolerogenic doses of levan, human gamma globulin, and soluble capsular polysaccharide antigen. When these same antigens were administered in immunogenic form, no suppressor cell activity was detected. Therefore, the suppressive mechanism was common to models of immunologic tolerance and was not unique to cryptococcal disease or cryptococcal capsular polysaccharide antigen. The phagocytosis-inhibiting lymphokine produced by the suppressor cell population completely inhibited the phagocytic activity of only a portion of peritoneal exudate cells. Other macrophages in the population were not totally inhibited but exhibited a reduction in the number of yeast cells engulfed.

The regulation of polyclonal immunoglobulin synthesis by FcR+ and FcR- monocyte subsets

FcR+ and FcR- monocyte subsets were added to the pokeweed mitogen (PWM) or Staphylococcus aureus Cowan I-stimulated cultures of peripheral blood mononuclear cells (PBMC) or to PBMC depleted of monocytes. The numbers of immunoglobulin-secreting cells (ISC) and cells with intracytoplasmic immunoglobulins (PC) were evaluated 6 days later. The addition of FcR- subset increased the number of ISC in cultures of PBMC stimulated with PWM and reconstituted the response of monocyte depleted PBMC. In contrast, FcR+ monocytes suppressed PWM-induced response and, when added in high dose, also that induced by S. aureus. The FcR+ monocytes suppressed the response by inhibition of immunoglobulin secretion but not the development of PC. This suggests that FcR+ monocytes may modulate humoral response by preferential inhibition of the final differentiation of B lymphocytes into ISC.

A 3 beta-cholestanyl-containing mono-benzylpenicilloyl oligoamide and peptide suppress anti-benzylpenicilloyl antibody formation in mice

A long-chain linear mono-benzylpenicilloyl (BPO) oligoamide and a succinoylated mono-BPO decalysine were tested in BALB/c mice for suppression of IgE and IgG1 antibody formation. Both compounds were available with either a free C-terminal end or were C-terminally linked to a hydrophobic 3 beta-cholestanyl residue. Only the sterol-containing derivatives suppressed hapten-specific IgE and IgG1 responses. Substantial suppression was obtained when the compounds were administered before primary or secondary, but not later immunizations. In an adoptive cell transfer experiment, spleen cells from tolerized animals actively suppressed anti-BPO IgE antibody formation of immune spleen cells. This effect was reversed by pretreatment of the tolerized spleen cells with anti-Lyt-2.2 antibody plus complement. The requirement for macrophages in the induction of T suppressor cells was demonstrated by injecting antigen-pulsed macrophages into naive recipients; upon immunization, only mice treated with tolerogen-pulsed macrophages showed suppressed anti-BPO IgE responses. It is suggested that lipid modification of antigens alters their processing and presentation by macrophages in a manner that leads to the induction of T suppressor cells. Injection of the cholestanyl derivatives into passively sensitized guinea pigs elicited anaphylactic reactions. By immune precipitation analysis and molecular weight estimation, these derivatives were shown to form micelles in aqueous solution. Therefore, the anaphylactic response appeared to be due to their behavior as multivalent antigens.

Regulation of thymocyte proliferative response by macrophage-derived prostaglandin E2 and interleukin 1

The antagonistic effects of macrophage (M phi)-derived interleukin 1 (IL 1) and prostaglandin E2 (PGE2) on thymocyte proliferation were uncoupled and studied. Elimination of PGE2 from the culture medium of prostaglandin E2-stimulated M phi was achieved by dialysis of the media or by indomethacin treatment of the M phi. IL1 secretion appears to be PGE2 independent. Experiments using exogenous PGE2 revealed a quantitative relationship between the two monokines. PGE2 (1.25 ng/ml) reduced the proliferative effect of IL 1 produced by 1.5 X 10(5) peritoneal M phi to 50%. This PGE2 dose increased significantly the levels of intracellular cAMP. Separation of peritoneal exudate M phi on a bovine serum albumin discontinuous gradient demonstrated that the main part of PGE2 synthesis was in a fraction of lower density, large M phi, whereas IL 1 activity was detected in all tested fractions.

Capacity of different cell types to prime in vivo for secondary in vitro cytotoxic T-cell responses against non-major-histocompatibility antigens

The capacity of several types of cell preparations to induce in vivo a state of memory for a secondary in vitro cytotoxic response against non-major-histocompatibility antigen was markedly reduced (on a per cell basis) by uv-irradiation. This indicated that memory induction requires metabolically active stimulator cells. An "adherent cell preparation" (AC) that was enriched for dendritic cells was among the most effective memory-inducing cell populations; but concanavalin A-activated nylon-wool-nonadherent spleen cells (Con A-NWT) or concanavalin A-activated unfractionated spleen cells (Con A-spl) were on the average equally effective. Normal unfractionated spleen cells (spl) or nonactivated nylon-wool-nonadherent cells (NWT) were markedly less effective on a per cell basis. This pattern of stimulatory activity was in line with the relative stimulatory activity of these cell types in primary cytotoxic responses in the presence of interleukin 2 (IL-2) and also in line with the relative capacity to induce IL-2-dependent proliferation in H-2D-incompatible T-cell populations (cf. W. Dröge et al., J. Immunol. 132, 2749, 1984). These differences in the immunogenic potential and the requirement for metabolically active stimulator cells suggested that these cells stimulated the CTL system directly and not indirectly through antigen processing cells of the immunized host. Nevertheless, the secondary cytotoxic response after injection of low numbers of Con A-spl into H-2 heterozygous recipients, (BALB/c X BALB/b)F1, or into recipients with recombinant H-2 haplotype (A.J) was only preferentially but not exclusively restricted to the H-2 haplotype of the immunizing cell populations. Restriction was considerably more complete when AC were used for immunization.

T cell-mediated immunity in murine malaria. II. Induction of protective immunity to P. chabaudi by antigen-fed macrophages and antigen-educated lymphocytes

We previously described assay systems for generating antigen specific proliferating T cells to P. chabaudi antigens. In the present study we examine whether the various sensitization approaches confer immunity against a cloned virulent strain IP-PCI of P. chabaudi. We present data indicating that effective specific protective immunity can be induced through P. chabaudi antigen fed macrophages and antigen educated spleen cells (initiator lymphocytes). The expression of this protective immunity is proposed to depend on (a) antigen presentation and/or accessory function of macrophages and (b) the subsequent activation of T cell functions related to protection. Indeed analysis of different macrophage populations revealed a correlation between the expression of Ia molecules and IL-1 secretion with their capacity to induce antigen specific T cells in vivo and subsequent protective immune mechanisms. Thus these results emphasize the critical functions of accessory cells in determining the outcome of malaria infections.

The origin of monocyte-macrophage heterogeneity: possible alternatives

The heterogeneity of the monocyte-macrophage population may be explained by several alternatives. In the present study we summarize our data and other studies concerning the variability and instability in the expression of markers and functions by monocytes and macrophages as well as the analysis of cloned populations of macrophages. We suggest the hypothesis that most macrophage heterogeneity may be due to reversible changes in a multifunctional population.

T-cell mediated immunity in murine malaria. I. Induction of T-cell dependent proliferative responses to Plasmodium chabaudi

To investigate the mechanisms of cell mediated immunity to malaria, we studied different systems to measure specific activation of T lymphocytes by P. chabaudi antigens. Mice were primed by subcutaneous administration of parasite antigens followed by co-cultivation of lymphocytes taken from the draining lymph nodes in the presence of the priming antigen. A marked proliferative response was observed which was shown to be antigen specific, T-cell mediated and accessory cell dependent. Continuous T-cell lines were propagated in culture by repetitive restimulation in the presence of antigen and accessory cells, followed by expansion in a conditioned medium containing T-cell growth factors. These lines could be induced to proliferate to the priming antigen only in the presence of syngeneic accessory cells thus indicating that H-2 restriction operates in the recognition of plasmodium antigens by T cells. We also induced parasite specific T cells by the use of an in vitro primary 'education' system. Lymphocytes from unprimed mice were sensitized on parasite-fed macrophages and were then injected subcutaneously into each hind foot pad of syngeneic animals. This led to recruitment of antigen-reactive cells which were assayed in vitro by the ability of lymphocytes taken from the draining popliteal lymph nodes to proliferate in response to the sensitizing antigen. In vivo immunization with Plasmodium antigen fed macrophages also signalled antigen specific T cells that recruited reactive T cells in the draining lymph nodes.

Phagocytic uptake of synthetic particles in blood leukocytes of fetal and newborn athymic nude mice

Monocytes are the main phagocytic element in the blood of fetal mice; monocytes of athymic nude mouse fetuses (gestation days 17-20) ingest in vitro, in full blood, significantly more synthetic (HEMA) particles than the monocytes of their euthymic littermates. Blood leukocytes of athymic fetuses also bear much higher densities of Fc receptors for IgG2B. The differences in phagocytic uptake in blood leukocytes disappear and the differences in Fc(IgG2B) receptors decrease at postnatal days 2 and 4.

Dichloromethylene diphosphonate (Cl2MDP) reduces natural killer (NK) cell activity in mice

Daily administration of dichloromethylene diphosphonate (Cl2MDP) to (C57BL/6 X DBA/2) F1 hybrid mice, from two days of age (10 mg of P/kg body weight), resulted in a marked impairment of natural killer (NK) activity of spleen cells against YAC-1 lymphoma cells. The suppressive effect increased with the duration of the treatment. Cessation of the treatment led to a rapid recovery (in 2 weeks) of NK activity while the osteopetrotic bone lesions persisted. Thus, the loss of natural killing cannot be explained by the simple reduction of bone marrow volume secondary to Cl2MDP-induced osteopetrosis. However, as NK cells are considered to be dependent on the bone marrow because they cannot be sustained by extramedullary production, a direct effect of Cl2MDP on the generation of NK cell precursors by the bone marrow was not excluded. Cl2MDP was not directly toxic to the fully differentiated splenic NK cells, since the addition of Cl2MDP to the in vitro assay (10(-5)-10 micrograms/ml) did not reduce cytotoxicity. These studies suggest that impairment of NK activity during Cl2MDP treatment may have clinical toxicologic implications since NK cells have been suggested to play an important role in natural host defenses against infection and neoplasia.

The instability of membrane markers expressed by human monocytes and macrophages in culture

Surface markers were tested on freshly isolated human monocytes and following their in vitro maturation to macrophages. The markers tested were HLA-DR antigens, receptors for the Fc of IgG and complement as well as membrane markers defined by monoclonal antibodies. The results revealed a dynamic expression of some of the markers on monocytes which was influenced by several variables. The expression of the markers was modulated by the presence of different sera, by treatment with lymphokines and interferon and following the in vitro maturation of monocytes to macrophages. The most unstable marker was found to be the HLA-DR, which was modulated by all these variables. The 63D3 was affected by different sera and culture supernatant, as well as following the maturation of monocytes to macrophages, but not by lymphokines and interferon. One of the markers, the Mac 120, was found to be relatively stable and did not change significantly following the maturation of monocytes to macrophages. The Fc and complement receptors were also stable in their expression under these conditions, but were probably partially blocked in the presence of human serum. These results indicated that at least some of the heterogeneity related to the monocyte population was probably not due to the occurrence of stable subsets of cells, but rather to reversible changes in marker expression.

In-vitro differentiation of human monocytes into mature macrophages during long-term cultures

A method is described which allows human peripheral blood monocytes from any given donor to differentiate in vitro into mature macrophages. About 90% of the starting monocytes are maintained during the long-term culture and are matured to macrophages. Thus cell loss is minimal and the resulting population of mature macrophages can be regarded as representative for all possible macrophage subpopulations present in peripheral blood. These cultures represent a standardized model for in-vitro studies on the role of mature macrophages in various immunological reactions.

Splenic B cells function as immunogenic antigen-presenting cells for the induction of effector T cells

Previous studies performed in our laboratory have revealed that an ordered, sequential, tricellular interaction is obligatory for the antigen-driven induction of a specific effector memory T cell. Thus, it was found that antigen-pulsed peritoneal macrophages signal, in spleen cells, the generation of antigen-specific initiator lymphocytes. These lymphocytes, following injection to syngeneic recipients, recruit, in the draining lymph nodes, "virgin" antigen-reactive T lymphocytes. Although the nature of the first and last cell in the interacting sequence was well characterized, the identity of the intermediary initiator splenic cell was obscure. Studies were carried out to characterize the nature of the splenic initiator cells. It was found that spleen cells from nu/nu, adult thymectomized and neonatal thymectomized, or spleen cells from normal donors which had been subjected to cytolysis using anti-Thy-1.2 antibodies in the presence of complement, did generate, following interaction with keyhole limpet hemocyanin (KLH)-fed macrophages, specific initiator cells. Carrageenan impairment of spleen macrophages did not affect the generation of initiator cells, nor did the depletion of dendritic cells from the spleen. On the other hand highly enriched B cell, but not highly enriched T cell populations, when seeded on KLH-pulsed macrophages, generated antigen-specific initiators, which, in vivo, recruited antigen-reactive T cells. It thus appeared that B lymphocytes can function as intermediary obligatory antigen-presenting cells and actively transfer immunogenic signals from peritoneal antigen-presenting cells to T lymphocytes. These findings may therefore suggest that antigen-specific B cells do not function solely as antibody-producing cells, but, once activated by macrophages, may control the induction and differentiation of some antigen-reactive T cell subsets. Thus, one can view the B cell as an important regulatory cell of both cellular and humoral immune functions. The significance of this observation with regard to Ir gene control at the level of B lymphocytes is discussed.

Defective induction of antigen-reactive proliferating T cells in B cell-deprived mice. II. Anti-mu treatment affects the initiation and recruitment of T cells

Mice injected from day of birth onwards with rabbit anti-mouse IgM (antim-mu) antibodies were found to be B cell-deficient and defective for the induction of antigen-reactive proliferating T cells (TPRLF). This defective induction was not due to the absence of circulating antigen-specific antibodies since the daily injections of such antibodies during exposure to antigen did not restore the ability of anti-IgM treated animals to generate TPRLF. Analyzing the cellular events implicated in the induction of virgin antigen-reactive T cells, anti-mu-treated mice manifested impairment of the three interacting cell types involved in the induction of TPRLF. Thus, peritoneal and splenic antigen-presenting cells from such animals were impaired in their capacity to signal a primary antigen-specific T cell reaction. Their splenic lymphocytes could not function as initiator cells in transferring immunogenic signals to recruit TPRLF in normal recipients. Potent antigen-specific splenic initiator cells failed to induce the recruitment of specific TPRLF in anti-mu-treated mice. The defective induction of TPRLF in anti-mu-treated mice may be due to a functional impairment of cells expressing membrane-bound IgM molecules which seemingly play a central role in the transfer of immunogenic signals for the recruitment of antigen-specific circulating T cells. We suggest that splenic B cells function as initiators in the transfer of antigen-induced signals from peritoneal antigen-presenting cells to T cells. These seems to be the primary targets of anti-mu treatment.

Macrophage functions in antimicrobial defense

In the present short review article, we discuss some recent findings regarding the role of macrophages in antimicrobial defense. Problems that are briefly considered include the production of monocytes and its modification in infectious diseases; the role of circulating monocytes and "fixed" macrophages in the blood stream; emigration of monocytes from the vascular bed and their accumulation at sites of microbial attack; role of macrophages in immune responses; stimulation and activation of these cells including secretory functions; attachment to their surface and phagocytosis of microorganisms; microbicidal activities of macrophages; their role in granuloma formation; inherited and acquired deficiencies of macrophage functions. Emphasis is placed on currently unresolved problems.

Relation between langerhans cells, veiled cells, and interdigitating cells

Tissue macrophages are bone marrow derived mononuclear cells which play an important role in the immune response, especially as antigen presenting cells. They comprise a heterogeneous population of cells with phagocytic activity. On morphological functional and cytochemical criteria it is likely that the Langerhans cell (LC) in the epidermis, the veiled cell (VC) in the afferent lymph and the interdigitating cell (IDC) in the thymus dependent area of peripheral lymphoid organs and the thymus medulla belong to a subpopulation of the macrophages. They are low phagocytic, Ia positive and are highly immunogenic. VC and IDC may contain Birbeck granules, the characteristic organelles of the LC, suggesting a relationship between these cell types. An epithelial micro-environment as present in the skin epidermis and the thymus is necessary for the induction of these granules, which appear to have no immunological significance. In a scheme the development from monocyte into LC or into VC and subsequently IDC is postulated. Probably VC transport antigen from the skin area via the afferent lymphatics into the draining lymph node. In the thymus dependent area of this organ they present this antigen to T cells and mature into IDC. IDC in the medullary area of the thymus may also be involved in antigen presentation to immunocompetent T cells. However, in this central lymphoid organ a function in instruction of helper T cells may not be excluded.

Mechanisms controlling differentiation and function of antigen-presenting macrophages

We have briefly reviewed our studies on the mechanisms controlling the differentiation and activation of peritoneal antigen-presenting cells. We demonstrated that the peritoneal population is composed of two main subsets of cells, only one of which participates actively in primary antigen presentation. The latter is missing in athymic mice and seems to differentiate under the influence of the shortlived, cortisone-resistant subpopulation of thymocytes. The maturation of the peritoneal macrophages is subjected also to an additional inducing effect, that of the spleen. Macrophages from splenectomized donors are impaired both with respect to antigen presentation to naive and to primed lymphocytes, and with respect to phagocytosis of "opsonized" bacteria. The mature antigen-presenting cell is subjected to activating signals deriving from the Fc-bound Ig molecule. This is mediated via a tetrapeptide, tuftsin, which is cleaved off the CH2 portion of the Ig and activates the immunogenic effect of the antigen-pulsed macrophage.