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Potential role of autophagy in the bactericidal activity of human PMNs for Bacillus anthracis

Girish Ramachandran1, Padmaja Gade2, Pei Tsai3

  • 1Center for Vaccine Development, University of Maryland School of Medicine, Baltimore, MD 21201, USA gramacha@medicine.umaryland.edu.

Pathogens and Disease
|October 2, 2015
PubMed

Insights

Human polymorphonuclear leukocytes (PMNs) poorly support Bacillus anthracis endospore germination but rapidly kill vegetative bacilli via autophagy. This non-free radical pathway offers new insights into anthrax pathogenesis and host defense mechanisms.

Area of Science:

  • Immunology
  • Microbiology
  • Cell Biology

Background:

  • Bacillus anthracis endospores cause anthrax by germinating into vegetative bacilli within host cells.
  • Macrophages and dendritic cells are known to interact with B. anthracis, but the role of polymorphonuclear leukocytes (PMNs) is less defined.
  • Understanding PMN interactions is crucial for comprehending anthrax pathogenesis and host defense.

Purpose of the Study:

  • To investigate the role of human PMNs in the initial interaction with Bacillus anthracis endospores and vegetative bacilli.
  • To elucidate the mechanisms by which PMNs control B. anthracis infection.

Main Methods:

  • Culturing of Bacillus anthracis 34F2 Sterne endospores and vegetative bacilli.
  • Incubation with non-activated human polymorphonuclear leukocytes (PMNs).
  • Assessment of bacterial germination and viability within PMNs.
  • Analysis of microbicidal activity, including investigation of reactive oxygen and nitrogen species.
  • Evaluation of autophagic pathway involvement through LC-3 conversion, beclin-1 expression, SQSTM1 degradation, and inhibitor studies (3-methyladenine).

Main Results:

  • B. anthracis endospores exhibited poor germination within non-activated human PMNs.
  • Human PMNs demonstrated rapid microbicidal activity against outgrown vegetative B. anthracis bacilli.
  • This bacterial killing was independent of superoxide and nitric oxide, suggesting a non-free radical pathway.
  • An autophagic mechanism was identified in PMNs, characterized by LC-3 conversion, beclin-1 expression, and SQSTM1 degradation.
  • The autophagic killing was confirmed by inhibition with 3-methyladenine.

Conclusions:

  • Human PMNs play a significant role in controlling B. anthracis infection by inhibiting spore germination and actively killing vegetative bacilli.
  • PMNs employ an autophagic pathway, independent of reactive oxygen and nitrogen species, for B. anthracis killing.
  • These findings reveal a novel bactericidal mechanism in PMNs, contributing to the understanding of innate immunity against anthrax.
Keywords:
B. anthracisautophagyneutrophilsphagocytosis

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