Activation with lipopolysaccharide induces macrophages to produce the enzymes arginase and nitric oxide (NO) synthase. Both enzymes use as a substrate the amino acid L-arginine, which can be either hydrolyzed by arginase to urea and ornithine or oxidized by NO synthase to NO and citrulline. NO is important in the bactericidal and cytotoxic activities of macrophages. An equivalent functional role of arginase and its products is not known. We tested the induction of arginase in bone marrow-derived macrophages by endogenous mediators that are known to induce NO synthase, such as interferon-gamma (IFN-gamma), or suppress the induction of this enzyme, such as interleukin (IL)-4, IL-10, and prostaglandin E2 (PGE2). We find that PGE2 and the TH2 cytokines IL-4 and IL-10 are potent inducers of arginase. In contrast, the TH1 cytokine IFN-gamma does not induce arginase. Simultaneous application of both types of mediators leads to reduced induction of both arginase and NO synthase. Exposure of macrophage cultures to inducers of NO synthase exhausts their ability to respond subsequently to inducers of arginase. Conversely, exposure of the cells to inducers of arginase exhausts their ability to respond subsequently to inducers of NO synthase. The results are consistent with a competition of both enzymes for their substrate, L-arginine, with a reciprocal inhibition in the induction of both enzymes, or a combination of both phenomena. The enzymes NO synthase and arginase appear to define two alternate functional states of macrophages, induced by TH1 and TH2 cytokines, respectively.

Interferon (IFN)-γ, a key immunoregulatory cytokine, has been thought to be produced solely by activated T cells and natural killer cells. In this study, we show that murine bone marrow– derived macrophages (BMMΦ) secrete large amounts of IFN-γ upon appropriate stimulation. Although interleukin (IL)-12 and IL-18 alone induce low levels of IFN-γ mRNA transcripts, the combined stimulation of BMMΦ with both cytokines leads to the efficient production of IFN-γ protein. The macrophage-derived IFN-γ is biologically active as shown by induction of inducible nitric oxide synthase as well as upregulation of CD40 in macrophages. Our findings uncover a novel pathway of autocrine macrophage activation by demonstrating that the macrophage is not only a key cell type responding to IFN-γ but also a potent IFN-γ–producing cell.

An alanyl-alanyl-phenylalanyl-7-amino-4-methylcoumarin-hydrolyzing protease particle copurifying with 26S proteasomes was isolated and identified as tripeptidyl peptidase II (TPPII), a cytosolic subtilisin-like peptidase of unknown function. The particle is larger than the 26S proteasome and has a rod-shaped, dynamic supramolecular structure. TPPII exhibits enhanced activity in proteasome inhibitor-adapted cells and degrades polypeptides by exo- as well as predominantly trypsin-like endoproteolytic cleavage. TPPII may thus participate in extralysosomal polypeptide degradation and may in part account for nonproteasomal epitope generation as postulated for certain major histocompatibility complex class I alleles. In addition, TPPII may be able to substitute for some metabolic functions of the proteasome.

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We have recently shown that viable Borrelia burgdorferi organisms induce a chronic infection associated with arthritis and carditis in severe combined immunodeficiency (scid) mice but not in immunocompetent mice. The disease is similar to that found in patients suffering from Lyme disease. We now show that B. burgdorferi-specific immune mouse sera as well as a monoclonal antibody to the spirochetal outer surface antigen A (31 kDa) but not monoclonal antibodies specific for the 41-kDa antigenic component of the periplasmic flagella are able to prevent (or mitigate) the development of the disease in scid mice when passively transferred at the time ofthe bacterial inoculation.

A small dose of the IgG1 fraction of anti-idiotypic antibody (aId1) raised in guinea pigs against a strain A/J antibody specific for streptococcal Group A carbohydrate sensitizes A/J mice against Group A streptococci. This is opposed to the previously established suppressive function of anti-idiotypic antibody of the IgG2 class (aId2). Correspondingly, aId1 but not aId2 is eliminated from the circulation in the way typical of an immunogenic molecule. However, the stimulatory component in the IgG1 fraction is not necessarily itself IgG1 antibody. Sensitization occurs in both B and helper T lymphocytes and is specific for Group A streptococci. In the B cell compartment sensitization is restricted to precursor cells expressing the idiotype. The concomitant activation of T helper cells therefore suggests that these cells make use of receptors with a similar or identical idiotype. Efficient sensitization by aId1 of both T and B cells is also demonstrated in strain C57L/J mice which upon immunization with Group A streptococci express a partially cross-reacting idiotype as a minor component. When such animals were primed with aId1, essentially all of the anti-carbohydrate antibody carried the partially cross-reacting idiotype.

The functional heterogeneity of the CD4+ T cell response to Plasmodium chabaudi has been evaluated. Using a limiting dilution assay system and a variety of assays to detect gamma-interferon (IFN-gamma), interleukin-2 (IL-2), IL-3, and T helper (Th) cells for malaria-specific antibody production, the precursor frequencies of P. chabaudi-reactive T cells have been calculated. The patterns of lymphokines produced by individual microcultures of the limiting dilution assay generally supported the idea of two functionally distinct CD4+ subsets: one which produces IFN-gamma and IL-2 (Th1) and one which is an effective helper cell for antibody production (Th2). However, it could not be determined whether the overlapping functions observed in some cultures represented T cells which could produce all factors or separate clones which were developing in the same wells. During the first 14 days of an erythrocytic infection of P. chabaudi the predominant T cell response was of the Th1-type. The frequency of these cells decreased after 14 days. By 3 weeks after infection the CD4+ T cell response was characterized by Th2 cells, as defined by their ability to act as helper cells in the production of malaria-specific antibody. These data support the hypothesis that early clearance of P. chabaudi may be antibody-independent but that the final clearance mechanism coincides with the appearance of helper cells and antibody.