A Cardinal Role for Cathepsin D in Co-Ordinating the Host-Mediated Apoptosis of Macrophages and Killing of Pneumococci

Martin A Bewley,Helen M Marriott,Moira K B Whyte,Sheila E Francis,David H Dockrell,Benny Chain,Calogero Tulone,Timothy J Mitchell,Guido Kroemer,Robert C Read

doi:10.1371/journal.ppat.1001262

Abstract

The bactericidal function of macrophages against pneumococci is enhanced by their apoptotic demise, which is controlled by the anti-apoptotic protein Mcl-1. Here, we show that lysosomal membrane permeabilization (LMP) and cytosolic translocation of activated cathepsin D occur prior to activation of a mitochondrial pathway of macrophage apoptosis. Pharmacological inhibition or knockout of cathepsin D during pneumococcal infection blocked macrophage apoptosis. As a result of cathepsin D activation, Mcl-1 interacted with its ubiquitin ligase Mule and expression declined. Inhibition of cathepsin D had no effect on early bacterial killing but inhibited the late phase of apoptosis-associated killing of pneumococci in vitro. Mice bearing a cathepsin D−/− hematopoietic system demonstrated reduced macrophage apoptosis in vivo, with decreased clearance of pneumococci and enhanced recruitment of neutrophils to control pulmonary infection. These findings establish an unexpected role for a cathepsin D-mediated lysosomal pathway of apoptosis in pulmonary host defense and underscore the importance of apoptosis-associated microbial killing to macrophage function.

Highlights

Macrophages are essential for the maintenance of tissue homeostasis, as they remove dying and dead cells [1]
We have previously demonstrated that macrophage apoptosis during pneumococcal infection is caspase-dependent [18] and caspase activation has been reported to trigger lysosomal membrane permeabilization (LMP) [29]
These results indicate that pneumococcal infections cause impairment of lysosomal acidification and/or LMP

Summary

Introduction

Macrophages are essential for the maintenance of tissue homeostasis, as they remove dying and dead cells [1]. Macrophages must coordinate the innate response to microorganisms that penetrate sterile environments such as the lower respiratory tract [2,3]. To accommodate their opposing roles in long-term tissue homeostasis and short-term immune responses, tissue macrophages, such as alveolar macrophages, are long-lived in the basal state [4,5], yet can activate a variety of death pathways upon pathogen encounter [6]. Macrophage function is regulated by induction of apoptosis during pneumococcal infection [8,10]. During pneumococcal infection Mcl-1 downregulation is regulated post-transcriptionally with evidence of enhanced ubiquitination [12]

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Results

Conclusion