Antimicrobial proteins act as “alarmins” in joint immune defense

Abstract

The production of antimicrobial peptides and proteins by multicellular organisms is an important means of host defense (1). Since the demonstration in 1965 of lysozyme as a microbicidal protein against gram-positive bacteria (2), hundreds of antimicrobial proteins have been characterized from various multicellular organisms. Recently, the participation of several antimicrobial peptides in antimicrobial defense has been validated using transgenic or knockout mouse models (3–5). In general, antimicrobial peptides are geneencoded, with most synthesized as precursors and processed by proteases into active mature forms (most often consisting of the C-terminal portion of the translated products) before or after release from producing cells. Antimicrobial peptides are diverse in terms of size; amino acid composition, sequence, and structure; and activity. However, common to all antimicrobial peptides is their rapid cytostatic or cytocidal effect on a broad spectrum of bacteria, fungi, some parasites, viruses, and certain tumor cells. The mechanisms of the inhibitory effect of antimicrobial proteins include depriving ions essential for microbial survival, degrading structural components (e.g., peptidoglycan) of microorganisms, and disrupting the integrity of target cell membrane by punching pores in the membrane or by perturbing membrane integrity. In humans, scores of antimicrobial peptides have been characterized at the protein level (Table 1). Human defensins are divided into 2 subclasses, and , based on the differences in the positions and 3 disulfide bonds of their 6 cysteine residues (1,6). Of the 6 human -defensins identified, 4 are human neutrophil granule components termed human neutrophil peptides 1–4 (HNP-1–4). The other 2 human -defensins (HD-5 and HD-6) are predominantly present in the secretory granules of Paneth cells, a type of epithelial cell located at the base of small intestinal crypts. Four human -defensins (HBD-1–4), recently characterized at both the gene and protein levels, are produced by epidermal keratinocytes and/or epithelial linings of the respiratory, urogenital, and digestive tracts. HBD-1 is constitutively expressed, whereas the production of HBD-2–4 can be strongly induced in response to proinflammatory stimuli, such as microbial products (e.g., lipopolysaccharide [LPS]) and/or cytokines (e.g., interleukin-1 , tumor necrosis factor [TNF ], or interferon). Similar to defensins, other human antimicrobial proteins are also produced predominantly by various types of leukocytes and/or epithelial cells. Thus, antimicrobial peptides are predominantly located either in professional phagocytic executioners (leukocytes) or at the body surfaces most likely to initially encounter potential microbial invaders. Do antimicrobial peptides play a role in antimicrobial immune defense against microorganisms accidentally penetrating into tissues that are not normally populated by leukocytes or epithelial cells, such as joints? In this issue of Arthritis & Rheumatism, Varoga et al report that chondrocytes in the articular cartilage produce a battery of antimicrobial peptides that could potentially contribute to combating microbial invaders in the joint (7). Clinically, bacteria can gain access to joints as a result of hematogenous seeding during a bacteremic episode or by direct introduction due to penetrating trauma or joint surgery. Nearly every bacteThe content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the US Government. Supported in part by the National Cancer Institute, NIH (contract no. N01-C0-12400). De Yang, MD, PhD: Science Applications and International Corporation–Frederick, Center for Cancer Research, National Cancer Institute, NIH; Joost J. Oppenheim, MD: Laboratory of Molecular Immunoregulation; National Cancer Institute, NIH, Frederick,Maryland. Address correspondence and reprint requests to De Yang, MD, PhD SAIC–Frederick, National Cancer Institute, NIH, Frederick, MD 21702-1201. E-mail: dyang@mail.ncifcrf.gov. Submitted for publication March 16, 2004; accepted in revised form August 2, 2004. Arthritis & Rheumatism