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Macrophage migration inhibitory factor (MIF) is a key mediator of the innate immune system and plays a crucial role in the host response to bacterial infections. Its role in immunity to intracellular pathogens has not been well studied. Here, we show that MIF released by infected human macrophages inhibits the growth of virulentMycobacterium tuberculosis.

Mycobacterium avium is a common opportunistic pathogen in immunocompromised patients such as those infected with human immunodeficiency virus. Although M. avium is an intracellular organism replicating predominantly in macrophages, disseminated M. avium infection is seen in AIDS patients with CD4 + cell counts of M. avium . To determine whether CD40-CD40 ligand (CD40L) interactions play a role in M. avium infection, we studied the ability of CD40L to restrict M. avium replication in human monocyte-derived macrophages (MDM) in vitro. MDM were infected with M. avium and cocultured with CD40L-transfected 293 cells for 7 days. Intracellular growth of M. avium in these MDM was assessed by colony counting. CD40L-expressing cells inhibited growth of M. avium in MDM by 86.5% ± 4.2% compared to MDM cultured with control cells. These findings were verified by assays using purified, soluble recombinant human CD40L (CD40LT). CD40LT (5 μg/ml) inhibited intracellular growth of M. avium by 76.9% ± 18.0% compared to cells treated with medium alone. Inhibition by CD40LT was reduced by monoclonal antibodies (MAbs) against CD40 and CD40L. The inhibitory effect of CD40LT was not accompanied by enhancement of interleukin-12 (IL-12) production by M. avium -infected MDM, while CD40L-expressing cells stimulated IL-12 production by these cells. Treatment of M. avium -infected mice with MAb against murine CD40L resulted in recovery of larger numbers of organisms (0.8 to 1.0 log) from the spleens, livers, and lungs of these animals compared to infected mice which received normal immunoglobulin G. These results indicate that CD40-CD40L signaling may be an important step in host immune response against M. avium infection.

Strains of the Mycobacterium avium complex (MAC) yield opaque and transparent colonial variants when cultivated in vitro. The transparent variants are more virulent than the opaque for animals, but little is known about the respective roles of these colonial variants in humans. To assess which variant infects humans, various blood fractions from eight patients with MAC bacteremia were plated directly onto 7H10 agar. In cell fractionation studies, all the M. avium complex CFU were associated with leukocytes and none were found free in plasma. All colonies on the primary culture plate exhibited the transparent phenotype. However, during subculture in 7H9 broth or on Lowenstein-Jensen agar, opaque variants appeared in seven of eight strains. Isogenic pairs of transparent and opaque variants were prepared and used to infect in vitro human monocyte-derived macrophages from healthy seronegative individuals. Transparent variants invariably grew inside macrophages, but only one of seven opaque variants did so. These observations indicate that the bacteremia of M. avium complex in acquired immunodeficiency syndrome patients consists exclusively of the transparent variants, perhaps because these variants are able to multiply inside macrophages. In contrast, opaque variants appear after in vitro subculture and are controlled by human macrophages, consistent with their reduced virulence in animals.

In the presence of temporary obstruction (20 h), ascending Escherichia coli urinary infection leads to irreversible acute exudative pyelonephritis (AEP) in rats. The purpose of the present study was to investigate the early inflammatory events which take place in response to the presence of bacteria in the kidney parenchyma and lead to the development of AEP. Rats were given indomethacin before and during the obstructive phase of kidney infection and were sacrificed at different times thereafter. Although renal infection (as defined by bacterial counts) was equally frequent (76%) and severe in indomethacin-treated and control rats sacrificed at the end of the obstructive period, it was found that the incidence of AEP (as defined by the inflammatory response of the kidney elicited by bacteria) 2 days after removal of the obstruction was significantly reduced from 74% in controls given water to 48% in indomethacin-treated animals (P = 0.02). Rat kidneys without AEP had bacterial counts of 10(2)/g. Since indomethacin apparently had no direct antibacterial activity against E. coli and no effect on urine osmolalities, it is likely that the reduction in the incidence of AEP and the concomitant eradication of bacteria after removal of the obstruction was due to an effect of indomethacin that is related to the renal response to infection. This was possibly due to decreased inflammation, as indicated by the fact that when pyelonephritis developed in indomethacin-treated rats it was less severe than in controls. These results suggest that if inflammation can be mitigated when bacteria are present in the kidney during obstruction, the bacteria may be cleared spontaneously once the normal urinary flow is restored.

Previous experiments with rats have suggested that pyelonephritic scarring after acute ascending Escherichia coli pyelonephritis partly results from excessive polymorphonuclear leukocyte (PMN) infiltration and activation in the kidney parenchyma. We have studied the role of PMN oxidative metabolism in generating tissue injury during acute pyelonephritis. Rats with acute pyelonephritis were treated with dapsone (25 mg/kg twice daily for 3 days), a compound known to prevent PMN oxidant damage. In vitro, levels of dapsone easily achieved in vivo inhibited myeloperoxidase (MPO)-mediated reactions involving the oxidation of halides to reactive cytotoxic hypohalites (such as MPO-mediated iodination and luminol-enhanced chemiluminescence). In contrast, dapsone had no effect on superoxide production, lysosomal enzyme release, or bacterial killing by activated PMN. In vivo, dapsone treatment had no significant effect on acute pyelonephritis with respect to (i) bacterial counts, (ii) inflammatory swelling, and (iii) PMN infiltration. However, dapsone-treated animals sacrificed 2 months after acute pyelonephritis had a 65% reduction of renal scars when compared with controls. Since dapsone had no antibacterial effect, this protection is compatible with the hypothesis that dapsone prevented oxidant-generated tissue injury due to the extracellular release of the MPO system by activated PMN during acute suppurative pyelonephritis.