Role of cytokines in the maturation and function of macrophages. Effect of GM-CSF and IL-4

Noncapsulated Klebsiella pneumoniae bearing mannose-containing O antigens is rapidly eradicated from mouse lung and triggers cytokine production by macrophages following opsonization with surfactant protein D

To better understand the relationship between the surface polysaccharides of pulmonary pathogens and components of the lung innate immune system, we employed selected serotypes of Klebsiella pneumoniae expressing distinct capsular polysaccharides and/or O antigen in a murine model of K. pneumoniae infection. In addition, we examined the effect of surfactant protein D (SP-D) on the cytokine response of human monocyte-derived macrophages to these serotypes in vitro. Noncapsulated mannose-containing O3 serotypes (K50/n and K55/n), which react efficiently with SP-D in vitro, triggered high levels of interleukin-1beta (IL-1beta) and IL-6 production. In vivo, they were more efficiently cleared from the lungs of mice but not from macrophage-depleted mice. They also were more efficiently internalized by alveolar macrophages in vivo. In contrast, galactose-containing O1 serotypes (K2/n and K21a/n), which interact poorly with SP-D, exhibited significantly lower cytokine production and less efficient pulmonary clearance and were ineffectively internalized by alveolar macrophages. These findings are consistent with in vitro results showing that production of IL-1beta and IL-6 mRNA and IL-6 protein by human macrophages exposed to mannose-bearing Klebsiella O serotypes is significantly increased by SP-D. Thus, survival of inhaled bacteria in the lung depends partially on the lipopolysaccharide structure of the bacteria and their interactions with innate immunity components. We speculate that an imbalance of host SP-D and therefore cytokine levels may result in high susceptibility of the host to the pathogen.

Peroxisome-proliferator-activated-receptor gamma (PPARgamma) independent induction of CD36 in THP-1 monocytes by retinoic acid

Retinoic acid (RA) has been shown to regulate cellular growth and differentiation of a variety of cell types, including cells of the myelomonocytic lineage. We used the monocytic leukaemia cell line THP-1, which differentiates to macrophages in response to phorbol 12-myristate 13-acetate (PMA), to investigate the regulation by RA of genes in the scavenger receptor type B family (CD36) in human monocyte/macrophages. Reverse transcription-polymerase chain reaction and flow cytometry demonstrated that, like PMA and the natural peroxisome-proliferator-activated receptor-gamma (PPARgamma) ligand 15d-PGJ2, RA induced CD36 gene expression in these cells. Moreover, RA plus 15d-PGJ2 further enhanced CD36 protein and mRNA levels over that seen with the RA or PPARgamma compounds alone. The PPARgamma antagonist GW9662 was shown to block completely PPARgamma-ligand induction of CD36 gene expression, but had little effect on the action of RA. Our data indicated that RXR- and RAR-specific ligands (LG153 and TTNPB, respectively) were each alone able to increase CD36 mRNA and surface protein levels. By using calphostin C, a specific protein kinase C (PKC) inhibitor, we demonstrated that induction of CD36 by PMA, as well as by PPARgamma and RXR ligands were dependent upon PKC activation. In contrast, activation of CD36 through the RAR pathway was not affected by inhibition of PKC activity. Taken together, these data demonstrate that RA can up-regulate CD36 expression in human monocytes/macrophages. This regulation appears to be predominantly mediated through the RAR/RXR pathway of action and, unlike previously described methods of CD36 modulation, is independent of PPARgamma and PKC signalling. This study suggests a possible role for RA in physiological processes involving the scavenger receptor function in cells of the monocyte/macrophage lineage.

PPARgamma is not a critical mediator of primary monocyte differentiation or foam cell formation

In the present report we clarify the role of PPARgamma in differentiation and function of human-derived monocyte/macrophages in vitro. Rosiglitazone, a selective PPARgamma activator, had no effect on the kinetics of appearance of monocyte/macrophage differentiation markers or on cell size or granularity. Depletion of PPARgamma by more than 90% using antisense oligonucleotides did not influence accumulation of oxidized LDL or prevent the upregulation of CD36 that normally accompanies oxLDL treatment. In contrast, PPARgamma depletion reduced the expression of ABCA1 and LXRalpha mRNAs. Metalloproteinase-9 expression, a marker of atherosclerotic plaque vulnerability, was suppressed by rosiglitazone. We conclude that activation of PPARgamma does not affect monocyte/macrophage differentiation. In addition, PPARgamma is not absolutely required for oxLDL-driven lipid accumulation, but is required for full expression of ABCA1 and LXRalpha. Our data support a role for rosiglitazone as a potential directly acting antiatherosclerotic agent.