United States Army Medical Research Institute of Infectious Diseases, Frederick, Maryland, United States of America.
Institute of Human Virology, University of Maryland Biotechnology Institute, Baltimore, Maryland, United States of America.
PLoS Pathog. 2022 Sep 29;18(9):e1010851. doi: 10.1371/journal.ppat.1010851. eCollection 2022 Sep.
During infection, Bacillus anthracis bacilli encounter potent antimicrobial peptides (AMPs) such as defensins. We examined the role that B. anthracis capsule plays in protecting bacilli from defensins and other cationic AMPs by comparing their effects on a fully virulent encapsulated wild type (WT) strain and an isogenic capsule-deficient capA mutant strain. We identified several human defensins and non-human AMPs that were capable of killing B. anthracis. The human alpha defensins 1-6 (HNP-1-4, HD-5-6), the human beta defensins 1-4 (HBD-1-4), and the non-human AMPs, protegrin, gramicidin D, polymyxin B, nisin, and melittin were all capable of killing both encapsulated WT and non-encapsulated capA mutant B. anthracis. However, non-encapsulated capA mutant bacilli were significantly more susceptible than encapsulated WT bacilli to killing by nearly all of the AMPs tested. We demonstrated that purified capsule bound HBD-2, HBD-3, and HNP-1 in an electrophoretic mobility shift assay. Furthermore, we determined that the capsule layer enveloping WT bacilli bound and trapped HBD-3, substantially reducing the amount reaching the cell wall. To assess whether released capsule might also play a protective role, we pre-incubated HBD-2, HBD-3, or HNP-1 with purified capsule before their addition to non-encapsulated capA mutant bacilli. We found that free capsule completely rescued the capA mutant bacilli from killing by HBD-2 and -3 while killing by HNP-1 was reduced to the level observed with WT bacilli. Together, these results suggest an immune evasion mechanism by which the capsule, both that enveloping the bacilli and released fragments, contributes to virulence by binding to and inhibiting the antimicrobial activity of cationic AMPs.
在感染期间，炭疽杆菌杆菌遇到了强效的抗菌肽（AMPs），如防御素。我们通过比较完全毒性的包膜野生型（WT）菌株和同源性包膜缺陷的 capA 突变菌株，研究了炭疽杆菌包膜在保护杆菌免受防御素和其他阳离子 AMP 的作用。我们鉴定了几种能够杀死炭疽杆菌的人防御素和非人类 AMP。人α防御素 1-6（HNP-1-4、HD-5-6）、人β防御素 1-4（HBD-1-4）和非人类 AMP，防御肽、短杆菌肽 D、多粘菌素 B、乳链菌肽和蜂毒素都能够杀死包膜 WT 和非包膜 capA 突变的炭疽杆菌。然而，非包膜 capA 突变杆菌比包膜 WT 杆菌更容易受到几乎所有测试 AMP 的杀伤。我们证明了纯化的包膜可以结合 HBD-2、HBD-3 和 HNP-1，并在电泳迁移率变动分析中发生了变化。此外，我们确定了包围 WT 杆菌的包膜层结合并捕获了 HBD-3，从而大大减少了到达细胞壁的量。为了评估释放的包膜是否也可能发挥保护作用，我们在将 HBD-2、HBD-3 或 HNP-1 加入非包膜 capA 突变杆菌之前，先将其与纯化的包膜预孵育。我们发现，游离的包膜完全挽救了 capA 突变杆菌免受 HBD-2 和 HBD-3 的杀伤，而 HNP-1 的杀伤作用降低到与 WT 杆菌相同的水平。总之，这些结果表明，包膜通过与阳离子 AMP 结合并抑制其抗菌活性，从而逃避免疫，这有助于炭疽杆菌的毒力。
