Membrane vesicles shed by murine melanoma cells selectively inhibit the expression of Ia antigen by macrophages

We previously demonstrated that membrane vesicles shed by the F10 variant of the murine B16 melanoma cell line inhibited the induction by interferon-gamma (IFN) of murine macrophage immune response region-associated (Ia) antigen expression. In this paper we present evidence that the inhibition of macrophage Ia antigen expression is a selective effect of vesicles and characterize its temporal requirements. Membrane vesicles shed from F10 cells did not affect the expression of macrophage H-2K or H-2D antigens under conditions shown to profoundly inhibit Ia antigen expression. Similarly, the induction of plasminogen activator and interleukin 1 from macrophages was not inhibited by the vesicles. The vesicles did not measurably decrease total cellular RNA or protein synthesis. Macrophages were sensitive to the inhibitory effects of the vesicles during the induction and maintenance phases of Ia expression. Pretreatment of macrophages with vesicles before culture with IFN did not reduce the induction of Ia. The rate of decline of Ia expression after removal of IFN was unaffected by the presence of vesicles. Removal of vesicles from cultures of IFN-treated macrophages resulted in only a partial recovery of Ia expression, suggesting that the inhibition of Ia expression may be a slowly reversible process. The selective and partially reversible inhibition of Ia expression by vesicles shed from the plasma membrane of tumor cells is a possible mechanism whereby tumor-bearing hosts may become immunocompromised.     

Species-dependent regulation of monocyte/macrophage Ia antigen expression and antigen presentation by prostaglandin E

The expression of Ia antigen by various murine and human macrophage populations and the ability of prostaglandins of the E series to regulate Ia antigen expression were explored. Monocytes and macrophages from human and murine populations demonstrated a dichotomy in the expression of Ia antigen. Both human monocytes and macrophages expressed elevated levels of Ia antigen compared to their murine counterpart. Murine macrophages appear to express elevated levels of Ia antigen only when actively interacting with T lymphocytes in vivo or with lymphokines in vitro. Prostaglandins of the E series can suppress murine macrophage Ia antigen expression, but have little effect on the expression of Ia antigen by human monocytes and macrophages. Also, prostaglandins of the E series do not modulate the ability of human monocytes to present antigen to autologous lymphocytes when studied over a broad concentration range. These data suggest that prostaglandin E compounds do not profoundly affect human monocyte/macrophage Ia antigen expression or human monocyte antigen presenting activity.     

B cell stimulatory factor-1 (interleukin 4) activates macrophages for increased tumoricidal activity and expression of Ia antigens

Macrophages are activated by lymphokines (LK) to kill tumor cell and microbial targets. Interferon-gamma (IFN) is the major LK activity in conventional, antigen or mitogen-stimulated spleen cell culture fluids for induction of these macrophage effector functions. In view of the recent demonstration that murine macrophage-like cell lines have receptors for B cell stimulatory factor-1/interleukin 4 (BSF-1), a possible role for BSF-1 in regulation of macrophage function was considered. In this communication, thioglycollate-elicited murine peritoneal macrophages were shown to express about 2300 high affinity (Ka approximately 2 X 10(10) M-1) BSF-1 receptors/cell. Peritoneal macrophages treated with purified, T cell-derived BSF-1 developed potent tumoricidal activity against fibrosarcoma target cells. The concentration of BSF-1 that induced 50% of maximal tumor cytotoxicity was 38 +/- 4 U/ml for seven experiments; similar dose-responses were observed with recombinant BSF-1. That BSF-1 dose-responses for induction of macrophage-mediated tumor cytotoxicity were not affected by 5 micrograms/ml polymyxin B suggested that contaminant endotoxins played little or no role in cytotoxic activity. BSF-1 alone (less than or equal to 500 U/ml) was not directly toxic to tumor cells or macrophages. Macrophage tumoricidal activity induced by BSF-1 but not by IFN was inhibited greater than or equal to 90% with monoclonal anti-BSF-1 antibody. BSF-1 induced Ia antigen expression on peritoneal macrophages and increased (twofold to threefold) FcR(II)-dependent binding of murine IgG immune complexes to bone marrow-derived macrophages (greater than 98% macrophages). Based on these findings, it was concluded that BSF-1 is a potent macrophage activation factor.     

Turnover and fate of I-Ak antigen on the murine macrophage cell surface

The macrophage plasma membrane is a major site of the cell's activities, including phagocytosis, antibody-dependent cellular cytotoxicity, and antigen presentation. To present antigen, the expression by the macrophage of immune region-associated (Ia) antigen is required. The turnover and fate of this cell surface constituent was studied in macrophages cultured with lymphokine or recombinant interferon-gamma. Surface-labeled subregion I-Ak antigen was lost from the cell surface at a rapid rate, with a half-life of approximately 24 hours. However, the shedding of I-A antigen into the culture fluid was not detected. Therefore, the loss of I-A antigen from the macrophage surface is most likely by its degradation. Upon removal of lymphokine or interferon from macrophage cultures, I-A antigen expression declined, with an apparent half-life of 2 days.     

Bone marrow-derived macrophages: development and regulation of differentiation markers by colony-stimulating factor and interferons

To investigate the role of specific cytokines in the development of the fully mature macrophage, we have employed murine bone marrow cells that were grown in the presence of CSF-1, a colony-stimulating factor that has been shown to induce the proliferation and differentiation of macrophages from their precursor cells. The CSF-1 employed in these studies was partially purified to ensure removal of contaminating interferon (IFN) from the preparations. After 1 to 2 wk in the presence of the partially purified CSF-1, the adherent macrophages were removed from flasks enzymatically and were recultured at known densities in the absence of CSF-1. Cell surface antigens (Mac-1 and Ia) and Fc receptor capacity (as assessed by Fc-mediated phagocytosis) were examined as markers of macrophage differentiation. Basal levels of Fc receptor capacity and Mac-1 antigen were markedly influenced by exposure to CSF-1, and appear to be modulated by CSF-induced, macrophage-derived IFN. When the bone marrow-derived macrophages were exposed to exogenous IFN in the absence of CSF-1, they proved to be extremely inducible with respect to Fc-mediated phagocytosis (IFN-beta and rIFN-gamma) and Ia antigen expression (rIFN-gamma) when compared with thioglycollate-elicited macrophages. Thus, macrophage growth factors, such as CSF-1, promote macrophage maturation by inducing the production of autostimulatory signals, such as macrophage-derived IFN. In addition, exogenous cytokine stimuli, such as IFN-gamma, further amplify the differentiative potential of these cells. Bone marrow-derived macrophages, propagated under well-defined conditions and never exposed to eliciting agents, provide a powerful model for studying the role of cytokines, such as CSF-1 and IFN, in the differentiative pathway of macrophages.     

The handling of Listeria monocytogenes by macrophages: the search for an immunogenic molecule in antigen presentation

The activation of T lymphocytes for immunity to the intracellular pathogen Listeria monocytogenes requires that Ia-positive macrophages ingest the bacteria. The subsequent handling of Listeria by macrophages was examined in this report and related to antigen presentation to T cells. Macrophages pulsed with radiolabeled Listeria, besides releasing acid-soluble radioactivity--an indication of extensive catabolism of the Listeria-derived proteins--were also found to release acid-insoluble peptides. The rate of release of the peptides was not markedly affected by treatment with chloroquine, ammonia, or monensin and was independent of the state of activation and the level of Ia expression of the macrophage. The peptides were not associated with fragments of membranes and were represented by several molecular species. Listeria-derived peptides were also found associated with the macrophage plasma membrane. The membrane-associated peptides behaved like integral membrane proteins and could be released by proteases or detergents. Their expression was independent of the dose of Listeria and the level of Ia expression of the macrophage, and their presence could not be inhibited by protease inhibitors or chloroquine. The Listeria peptides released by the macrophages were very weakly immunogenic in a T cell proliferation assay. Purified plasma membranes from Listeria-pulsed macrophages, which contained membrane-associated Listeria peptides, were not immunogenic by themselves but could be reprocessed by additional macrophages to subsequently stimulate T cells. Trypsin treatment of Listeria-pulsed macrophages did not cause a significant reduction in their ability to stimulate T cells. No association was found between Ia molecules and either the membrane-associated or the released peptides with the use of several technical approaches. Hence, after internalization of Listeria, potentially immunogenic material can be found at the cell surface as well as in the culture fluid. The release of soluble peptides is a clear indication that proteins can be recycled after their internalization in vesicles.     

Combined effects of tumor necrosis factor-alpha, prostaglandin E2, and corticosterone on induced Ia expression on murine macrophages

Ia expression is an important marker of macrophage functional capacity. IFN-gamma induces Ia expression on perhaps all murine macrophages, whereas IL-4, granulocyte-macrophage CSF, and CSF-1 induce Ia on restricted sets of macrophages. Inhibitors of expression include PGE2, glucocorticoids, and IFN-beta. TNF has been found to augment Ia expression on several macrophage lineage cell lines but to inhibit expression on murine peritoneal macrophages. Our study shows that TNF can have opposite effects on Ia expression (induced by IFN-gamma) on thioglycollate-elicited peritoneal macrophages, depending on the length of time cells are treated and on the presence of other modulators. In particular, TNF augmented early expression induced by IFN-gamma but inhibited later expression. And although TNF synergized with PGE2 to markedly inhibit Ia induction on these cells, it partially antagonized the inhibition by corticosterone and IFN-beta. TNF and PGE2 also synergized to inhibit Ia expression induced on bone marrow-derived and splenic macrophages by either IFN-gamma or IL-4. In contrast to their effect on Ia expression, TNF and PGE2 had opposite effects on expression of gamma 2a FcR in macrophages. TNF blocked the increase in FcR expression due to any combination of PGE2, IFN-gamma, and IFN-beta. However, TNF and PGE2 both increased expression of gamma 2a FcR on WEHI-3 cells. If the different effects of TNF reflect the differentiation states of macrophages, its effects on Ia and FcR expression may vary with the progression of an immune response.     

Persistent infection with herpes simplex virus type 1 in an Ia antigen-positive murine macrophage cell line

The interaction of herpes simplex virus type 1 (HSV-1) with murine macrophage cell lines was examined. The cell lines appeared to be moderately permissive for HSV-1 replication, though the yield of the virus was limited compared with that in Vero cells. Furthermore, the murine macrophage cell line SL-1, bearing Ia antigen, was persistently infected with HSV-1 for over one year, and was designated SL-1/KOS. Persistent infection could not be established in an Ia antigen-negative macrophage cell line, SL-4. In the SL-1/KOS culture, there was a small number of infected cells as revealed by infectious center assay. Treatment with monoclonal antibody against HSV-1 cured the persistent infection. Therefore maintenance of the persistent infection is considered to be due to a carrier culture consisting of a minority of infected cells and a majority of uninfected cells. In the SL-1/KOS cultures a low level of interferon (IFN) was found. When a large amount of exogenous recombinant murine IFN-beta (10(5)-10(6) international units/ml) was added to the culture, virus production diminished to undetectable levels. These results suggest that IFN plays an important role in the maintenance of persistent infection. In long-term persistently infected cultures, syncytium formation appeared and the virus from such cultures had a different DNA structure from that of the virus originally used for infection as revealed by restriction endonuclease analysis.     

Monoclonal antibody-mediated inhibition of interferon-gamma-induced macrophage antiviral resistance and surface antigen expression

A monoclonal antibody (MAb) with specificity for murine interferon-gamma (IFN-gamma) was used as a probe for studying the effect of recombinant IFN-gamma (rIFN-gamma) on antiviral activity, Fc receptor expression, and Ia antigen induction in macrophages. Cultures of C3H/HeJ peritoneal exudate macrophages were used to allow direct comparison of all three functions in the same target cell system. Our data provide two major findings: the efficacy of the MAb is very different depending on whether murine fibroblasts or macrophages are used as the target cell in the antiviral assay, i.e., greater than 20 to 100 times more MAb was required to block antiviral activity in macrophage cultures; and 10 to 50 times more MAb was required to inhibit Fc receptor vs Ia antigen expression in response to rIFN-gamma. These latter findings confirm and extend previous observations, which indicate that the induction pathways of two important differentiation markers by IFN-gamma may be dissociable.     

Antagonistic effect of interferon-beta on the interferon-gamma-induced expression of Ia antigen in murine macrophages

The cell surface expression of I region-associated (Ia) antigens by murine and human macrophages has been shown by investigators from a number of laboratories to be induced in a dose-dependent fashion by IFN-gamma, which is free of other lymphokines. The experiments described in this report demonstrate that fibroblast-derived IFN-beta exerts an antagonistic effect on IFN-gamma induced Ia expression in murine macrophages. Simultaneous addition of IFN-beta and IFN-gamma to peritoneal exudate macrophages results in decreased Ia expression when compared with macrophages treated with IFN-gamma only. Different sources of highly purified IFN-beta, as well as a recombinant human IFN-alpha (A/D Bgl; shown previously to be as active as IFN-beta in several other murine systems) acted in a similar antagonistic fashion to IFN-gamma-induced Ia induction. The down-regulation of Ia expression by IFN-beta is dose-dependent over a concentration range up to 100 U/ml. Time-course experiments indicated that for IFN-beta to down-regulate IFN-gamma-induced Ia, it had to be present either before stimulation with IFN-gamma or during the first 24 hr of simultaneous stimulation. Further experiments in which a highly specific antibody against IFN-alpha/beta was added to the cultures confirmed the findings of the time-course experiments. Inhibitors of the arachidonic acid pathway failed to reverse the effect of IFN-beta to reduce Ia antigen expression, which suggests that this inhibition is not prostaglandin mediated. Thus, these findings support a role for type I IFN as naturally occurring substances that negatively regulate the expression of class II molecules.     

Down-regulation of Ia expression on macrophages by sea star factor

Sea star factor (SSF), a protein of 39,000 Da isolated from the coelomocytes of Asterias forbesi, was found to inhibit the induction of Ia expression on murine macrophages by concanavalin A supernatants. Addition of SSF to cultured macrophages at the same time as the lymphokine preparation significantly reduced the percentage Ia+ cells after 5 days culture, compared to cultures given lymphokine only. Intraperitoneal injection of SSF also reduced the percentage Ia+ peritoneal exudate macrophages by three-fourths in Listeria-infected mice. Addition of the cyclo-oxygenase inhibitor, indomethacin, to macrophage cultures reversed this Ia-suppressive effect of SSF. Since macrophages from endotoxin-unresponsive and endotoxin-responsive mice were both sensitive to the Ia-inhibitory effect of SSF, the induction of arachidonic acid metabolism and the inhibition of Ia appear to be independent of the action of endotoxin. The SSF-induced down regulation of Ia expression may be a major factor in the suppression of primary immune responses to T-dependent antigens previously noted in studies with SSF.     

Inhibition of macrophage-induced antigen-specific T lymphocyte proliferation by poly I:C: strain and antigen independence

We have previously demonstrated that IFN-alpha/beta, poly I:C (an inducer of IFN-alpha/beta), and IFN-gamma can inhibit the ability of KLH-pulsed peritoneal macrophages to induce proliferation of syngeneic, KLH immune T lymphocytes in CBA/J mice. In this study, we show that this IFN-induced immunosuppression is not restricted to CBA/J (H-2k) mice but is also seen in BALB/cJ (H-2d) mice. A similar inhibition of proliferation is observed with the KLH-specific T cell hybridoma BDK, 100, which requires KLH-pulsed macrophages for optimum proliferation and IL-2 production. The immunosuppression produced by IFN was also independent of the antigen employed. Inhibition of T lymphocyte proliferation was observed when casein, instead of KLH, was used to immunize T cells and to pulse peritoneal macrophages in vivo. Utilizing KLH and casein, the antigen specificity of the inhibition was demonstrated. Therefore, the inhibition by the IFN-inducer poly I:C of macrophage-induced, antigen-specific T cell proliferation is not limited by H-2 type of the mice or to one antigen.     

Impairment of macrophage activation and granuloma formation by protein deprivation in mice.

Protein-calorie malnutrition predisposes to infection by intracellular pathogens, but the basis for this predisposition is unclear. We studied the effect of protein deprivation on mouse peritoneal macrophage function and on granuloma formation during infection by bacille Calmette-Gue?in (BCG). Injection of lipopolysaccharide (LPS) to induce inflammation elicited fewer peritoneal cells from mice fed a 2.5% protein diet than from mice fed an isocaloric chow in which protein calories constituted 24% of the total. LPS-elicited macrophages from protein-deprived mice demonstrated a reduction in spreading, total cell protein, cell lactate dehydrogenase, and release of superoxide anion (O2-) in response to stimulation. Priming in vitro by interferon (IFN)-gamma for enhanced release of O2- was also significantly impaired in protein-deprived mice. This defect was reversible by repletion with 24% protein diet for 10 days. Impairment of macrophage function in protein-deprived mice was further evidenced by an impaired capacity to express Ia antigen in response to IFN-gamma and by reduced production of IL-1 activity in response to LPS. Infection by BCG in protein-deprived mice was characterized by impaired granuloma development in liver, lungs, and spleen. Thus, in this model, protein deprivation significantly impaired macrophage activation, as assessed by morphologic, metabolic, and functional criteria. This impairment might compromise immune effector mechanisms dependent on macrophage activation, including rejection of intracellular pathogens.     

Tumor-induced alteration in macrophage accessory cell activity on autoreactive T cells

Using a tumor-model system, differences in the accessory cell capabilities on autoreactive T cells of splenic macrophages from normal and tumor-bearing hosts (TBH) were assessed in the syngeneic mixed lymphocyte reaction. Tumor development caused a drop in autoreactivity. At 0 and 7 days of tumor growth, no drop in reactivity occurred when TBH macrophages were used as accessory cells and L3T4+ autoreactive T cells from normal mice were used as responder cells. However, by day 14, there was a 32% drop in reactivity, and by day 21 only 22% of the T cell reactivity remained when TBH macrophages were used as accessory cells. Alterations in macrophage Ia antigen during tumor growth were first investigated as the potential cause of reduced autoreactivity. Before tumor growth (day 0) 59% of the splenic macrophages were found to be Ia+. Day-7 TBH macrophages showed no difference in Ia antigen expression when compared to day 0 macrophages. However, by day 14, TBH macrophages showed a 9% decrease, and by day 21 they showed a 36% decrease in the number which were Ia+. Concomitant with the decrease in the number of Ia+ cells was a decrease in the density of Ia antigen expression on day-14 and -21 TBH macrophages. In day-14 and -21 TBH macrophages, two populations were seen that were Ia+. The first had a 10%-20% decrease in Ia antigen expression per cell while the second population had a greater than 50% drop in Ia antigen expression per cell. By titrating and mixing TBH macrophages with normal host macrophages, we assessed whether they could actively mediate suppression of autoreactive T cells. A titratable suppressive phenomenon was demonstrated using day-21 TBH macrophages. In contrast, day-7 and -14 TBH macrophages titrated with normal host macrophages had no effect on the syngeneic mixed lymphocyte reactivity. Lastly, we investigated whether the macrophage-mediated suppression was caused by increased prostaglandin secretion. Addition of indomethacin to cultures increased autoreactive T cell reactivity stimulated by normal or TBH macrophages (59% and 99% increase, respectively). Although indomethacin reduced suppression mediated by TBH macrophages, autoreactivity did not return to levels induced by untreated or indomethacin-treated cells from a normal host. Taken together, the data suggested that tumor growth modulates the function of macrophage accessory cells with autoreactive T cells in at least two ways: by decreasing Ia antigen expression and by increasing suppressor activity.     

T cell induction of membrane IL 1 on macrophages

We have studied the role of T cells in the induction of a membrane-associated form of interleukin 1 (mIL 1) in murine macrophages. T helper cell clones and a T cell hybridoma induced macrophages to express mIL 1 after an antigen-specific, Ia-restricted interaction. Induction of mIL 1 was proportional to antigen concentration and was increased in the early course of the response in macrophages pretreated in culture with interferon-gamma. mIL 1 activity was detectable 4 hr after interaction with T cells. mIL 1 induction was inhibited by antibodies to either class II molecules or the T cell receptor. Two pathways of T cell-mediated mIL 1 induction could be defined. In the first, T cells, whose protein synthesizing capacity was completely eliminated by pretreatment with the irreversible protein synthesis inhibitor emetine, induced levels of mIL 1 expression indistinguishable from controls. In the second, T cells stimulated by paraformaldehyde-fixed macrophages in the presence of concanavalin A or antigen secreted a soluble factor that induced macrophage mIL 1 expression. Thus, it appears that T cells may induce macrophages to express mIL 1 both by direct cell-cell contact mediated through binding of T cell receptor to the Ia/antigen complex, and through the release of a lymphokine after activation. This lymphokine does not appear to be IL 2, IFN-gamma, BSF-1, or CSF-1.     

Regulation of constitutive and lymphokine-induced Ia expression by murine alpha-fetoprotein

alpha-Fetoprotein (AFP) has been shown to suppress a variety of immune responses in vitro. The immunosuppressive properties of AFP can be partly attributed to the ability of this protein to decrease the cell surface expression of Ia antigens on macrophages. The experiments described in this report define more precisely the regulatory effects of AFP on Ia expression. Using the "dendritic-like" cell line P388 AD2 and bone marrow-derived macrophages we have shown that AFP can suppress the constitutive expression of cell surface Ia antigens. This decrease is detectable on the cell surface 24 hr after the addition of AFP. In further experiments we also examined the effect of AFP on lymphokine-induced Ia expression. Our results show that AFP has no suppressive influence on the inductive phase of lymphokine-induced Ia antigen expression but can decrease elevated levels of Ia antigen subsequent to their induction.     

Role of bacterial hemolysin production in induction of macrophage Ia expression during infection with Listeria monocytogenes

The production of a hemolytic exotoxin (Hly) termed listeriolysin O (LLO) is a major determinant of the virulence of the Gram-positive bacterium Listeria monocytogenes. As determined by lethal inoculum size, LLO- strains of L. monocytogenes generally are several orders of magnitude less virulent than their LLO+ counterparts. The generation of protective anti-Listeria T cell immunity also has been shown to depend on the LLO phenotype of the bacteria present during primary infection, although the cellular basis of this observation is not known. The experiments described here address the role of LLO in regulation of the expression of class II MHC (Ia) molecules by murine macrophages. Because Ia expression by macrophages and other APC is thought to be a central factor in the generation of T cells specific for bacterial Ag, we have tested the hypothesis that the failure of LLO- strains to elicit anti-Listeria T cell responses might be secondary to an inability of these strains to stimulate increases in macrophage Ia levels. Our results show that the macrophage Ia response after i.p. injection of L. monocytogenes correlates strongly with the LLO phenotype of the bacteria. The presence of LLO+ organisms, even at very small numbers (as few as 10), elicits a striking increase in Ia expression by peritoneal macrophages. In contrast, even at very high numbers (up to 10(6) per mouse), LLO- bacteria fail to stimulate a strong Ia response. We also have analyzed macrophage Ia expression after injection of lysates of Escherichia coli expressing recombinant LLO protein. Similar to the results obtained with LLO+ and LLO- L. monocytogenes, we have observed Ia induction only with LLO+ lysates. Ia induction by this crude recombinant LLO preparation can be inhibited by cholesterol or heat. Furthermore, supernatants derived from cultures of LLO+ (but not LLO-) L. monocytogenes can cause Ia induction when administered via i.p. injection. Taken together, these findings suggest that the failure of macrophages to respond to LLO- organisms with an increase in Ia expression may be a major underlying cause of the failure of these bacteria to induce Listeria-specific protective T cell immunity. Furthermore, we propose that the induction of macrophage Ia expression in response to bacterial toxins such as Hly may represent one component of a set of early, innate immune mechanisms, and that this induction may provide a critical "bridge" to later, acquired, Ag-specific immune processes.     

Persistent expression of Ia-antigen on a subpopulation of murine resident peritoneal macrophages

The stability of Ia-antigen expression on murine resident peritoneal macrophages was assessed during the course of in vitro culture. Contrary to published findings with radioimmunoassays and immunofluorescence assays, the cultured cells bore Ia-antigen, as shown by their rosetting with sheep erythrocytes coupled with anti-Ia.2 monoclonal antibody. In support of this finding, cultured cells presented the copolymer of glutamine, alanine, and tyrosine (GAT) to GAT-primed T lymphocytes in an Ia-dependent manner. Thus, functional Ia antigen is present on cultured macrophages. Disappearance of the antigen after fixation of macrophages with either glutaraldehyde or paraformaldehyde, a routine procedure in the radioimmunoassays and immunofluorescence assays, explains its presumed absence on cultured cells.     

Suppression of major histocompatibility complex class I and class II gene expression in Listeria monocytogenes-infected murine macrophages

Macrophage cells play a central role during infection with Listeria monocytogenes by both providing a major habitat for bacterial multiplication and presenting bacterial antigens to the immune system. In this study, we investigated the influence of L. monocytogenes infection on the expression of MHC class I and class II genes in two murine macrophage cell lines. Steady-state levels of I-Aβ chain mRNA were decreased in both resting J774A.1 and P388D₁ macrophages infected with L. monocytogenes whereas reduction of H-2K mRNA was only observed in P388D₁ cells. In addition, L. monocytogenes suppressed induction of MHC class I and class II mRNAs in response to γ-interferon as well as the maintenance of the induced state in activated P388D₁ macrophages. Exposure to the non-pathogenic species L. innocua or a deletion mutant of L. monocytogenes, which lacks the lecithinase operon, did not cause a reduction in H-2K and I-Aβ mRNA levels nor suppress expression of Ia antigens. Inhibition of MHC gene expression may represent an important part of the cross-talk between L. monocytogenes and the macrophage that probably influences the efficiency of a T cell-mediated immune response and thus the outcome of a listerial infection.