探討 PrsA 作為治療 GAS 感染新標的之可能性

Abstract

化膿性鏈球菌 (Streptococcus pyogenes) 又稱為 A 型鏈球菌 (Group A Streptococcus,GAS) 能造成多種臨床感染症，包含輕度的表皮感染以及嚴重侵襲 性疾病，而反覆的 GAS 感染亦可能引發嚴重的自體免疫性疾病，如風濕熱、風 濕性心臟病和腎小球腎炎等。現今多以青黴素或其衍生物等抗生素來治療 GAS 感染，然而基於抗生素濫用，也逐漸有抗藥菌株出現。此外，目前並無安全有效 的 GAS 疫苗上市，研發中的疫苗主要以 GAS 的重要毒力因子作為標的，但皆尚 未進入 III 期臨床試驗。PrsA 為革蘭氏陽性菌中負責摺疊與穩定分泌蛋白的膜上 脂蛋白，我們實驗室先前發現 PrsA 對 GAS 中許多毒力因子的成熟、生物膜形 成、宿主細胞黏附能力及小鼠軟組織感染均極為重要。由於 PrsA 在不同血清型 的 GAS 中具有幾乎相同的基因序列，因此我們想進一步探討 PrsA 作為 GAS 治 療或者預防標的的可能性。我們發現 PrsA 在不同血清型的 GAS 皆有表現，PrsA 抗體除了能夠直接辨識到這些 GAS 表面的 PrsA 外，也不會與人心臟組織產生交 叉反應。此外，PrsA 抗體能有效促進嗜中性球的殺菌力，降低人類全血中 GAS 的存活率。事先給予小鼠 PrsA 抗體也能有效降低小鼠受到 GAS 侵襲性感染時的 死亡情形。雖然在 PrsA 抗原免疫的先導實驗中觀察到小鼠雖可以產生 PrsA 抗 體，卻未能降低小鼠受到 GAS 侵襲性感染時的死亡率，我們推測此結果可能與 施打疫苗時使用 alum 作為佐劑，產生偏向 Th2 型的免疫反應相關。總體而言， 我們的實驗結果顯示 PrsA 具有免疫原性且 PrsA 抗體具有促進嗜中性球殺菌的 能力並可以保護宿主去抵禦 GAS 感染。

Streptococcus pyogenes (Group A Streptococcus, GAS) can cause many clinical diseases ranging from superficial infections to severe invasive infections. Recurrent GAS infection is associated with occurrence of may even trigger severe autoimmune diseases such as rheumatic fever, rheumatic heart disease and glomerulonephritis. GAS infection is mainly treated with penicillin and its derivatives; however, antibiotic- resistant strains appear due to the misuse and overuse of antimicrobials. Currently, there are no safe and effective vaccines on the market. Most of the developing vaccine candidates are focused on the major surface proteins and virulence factors; however, none of these projects enter the phase III clinical trials. PrsA is a membrane-anchored lipoprotein responsible for the folding and stabilization of secreted proteins in Gram- positive bacteria. We have previously demonstrated that PrsA is critical for the virulence factors maturation, biofilm formation, host adhesion, and in vivo virulence in a murine soft tissue infection model. In this project, I aim to test whether PrsA may serve as a therapeutic target to treat GAS infection based on the prsA gene sequences are almost identical in different GAS serotypes. We found that PrsA is expressed in all the different GAS serotypes we tested, and the anti-PrsA antibody robustly recognizes surface-exposed PrsA. Notably, we did not observe the cross-reactivity of anti-PrsA antibody against the human cardiac myosin which is the major heart antigen recognized by the anti-M protein antibody. Moreover, the anti-PrsA antibody effectively promoted the neutrophil bactericidal efficacy and reduced the GAS survival in human whole blood. Mice received the anti-PrsA antisera prior to GAS challenge had significant higher survival rates compared to the mice received pre-immune sera. Despite all the PrsA-immunized mice produced high titers of PrsA-specific antibodies, we failed to observe the protective effects in the vaccinated animals upon invasive GAS challenge, which may be due to the biased Th2-prone immune responses induced by the Alum adjuvant used in this experiment. In summary, our results suggest that PrsA is immunogenic and can enhance the neutrophil bactericidal activities to protect the host against GAS infection.