HIF–1alpha regulates the bactericidal capacity of phagocytes

Abstract

Myeloid cells, i.e. monocytes/macrophages and polymorphnuclear neutrophils (PMN) are of pivotal importance for the body’s ability to defend itself against invading microorganisms. The main arenas of myeloid cell function, e.g. abscesses, inflamed tissues and wounds, are characterized by hypoxic conditions. This suggests a pronounced dependence of macrophages and PMN on the hypoxia-inducible. Here, we demonstrate that the principle mediator of cellular adaptation to low oxygen levels, the transcription factor HIF–1alpha, is essential for bactericidal activity of myeloid cells. Functional inactivation of HIF–1a in myeloid cells was achieved by the Cre-loxP-system. Mice homozygous for a floxed HIF–1a allele were intercrossed with animals expressing cre recombinase under the control of the lysozyme M-promoter. Deletion efficiency was analyzed by means of quantitative real-time PCR (Q-PCR). Bacterial challenge was performed by injecting live group A streptococcus (GAS, S. pyogenes) subcutaneous into the flanks of these mice. Mice with a conditional loss of HIF–1a displayed significantly lower body weight and greater skin ulcer size. Viability of GAS in blood and spleen samples of mutant animals was ten times higher than in wildtype littermates, where systemic detection of live bacteria was a rare event. In vitro studies demonstrated a 50–60% reduction of HIF–1a-deficient cells in intracellular killing of GAS. To investigate the underlying molecular mechanism, we characterized diverse anti-bacterial pathways and obtained in HIF–1a deficient myeloid cells: reduced iNOS expression and activity ultimately leading to reduced levels of NO, decreased enzyme activity of neutrophil elastase and reduced protein levels of the antimicrobial peptide CRAMP as well as decreased processing of this factor. These data strongly argue for a central role of HIF–1a in regulating bactericidal activity of myeloid cells. Inhibition of HIF–1a is thought to represent an attractive form of cancer therapy, given its central role in tumor progression via induction of hypoxia-triggered angiogenesis. Therefore, our results not only display a novel function for HIF–1a, but also point to systemic and possibly life-threatening bacterial infections as a side effect of therapeutic HIF–1a-inhibition.