被惡性瘧原蟲感染的紅細(xì)胞（iRBC）在早期的免疫反應(yīng)中，自然殺傷細(xì)胞（Natural killer cell，也譯作NK細(xì)胞）是被激活的，這表明NK細(xì)胞與先天性抗寄生蟲免疫反應(yīng)有關(guān)。然而，NK細(xì)胞是否會直接識別iRBC，刺激iRBC是否需要輔助細(xì)胞呈遞信號或是溶解性因子，這些問題目前還不明確。
近日，德國蒂賓根大學(xué)熱帶醫(yī)學(xué)研究所 Meral Esen等人發(fā)現(xiàn)：被瘧原蟲感染的紅細(xì)胞會通過宿主表達(dá)Hsp70來引起NK細(xì)胞釋放顆粒酶B，結(jié)果被感染的紅細(xì)胞衰亡。相關(guān)論文發(fā)表在3月15日的美國《公共科學(xué)圖書館—綜合》（PLoS One）上。
在近期的研究中，研究人員開展了關(guān)于“宿主細(xì)胞膜偶聯(lián)的熱休克蛋白70（Hsp70）會引起NK細(xì)胞對iRBC的毒性”的研究，這些NK細(xì)胞來自與瘧疾病人捐贈的NK細(xì)胞以及培養(yǎng)的NK92細(xì)胞系。對于表達(dá)于iRBC細(xì)胞膜的Hsp70和HLA-E，以及可能性的活化的NK細(xì)胞受體（包括NKG2C和CD94），該研究運用了流式細(xì)胞術(shù)以及免疫印記對它們進(jìn)行評估。
蛋白印跡、RT-PCR 以及elispot.html' target='_blank'>ELISPOT分析表明：顆粒酶B（GzmB）的產(chǎn)生及釋放都由沒有受過刺激的NK細(xì)胞，以及Hsp70肽（TKD）預(yù)先刺激的NK細(xì)胞所啟動。
研究結(jié)果表明，當(dāng)iRBC細(xì)胞上沒有HLA-E卻有Hsp70時，會促進(jìn)感染的宿主細(xì)胞成為了NK細(xì)胞調(diào)停的細(xì)胞毒性目標(biāo)。iRBC與NK細(xì)胞的則引起了GzmB的釋放，當(dāng)GzmB被攝取后，iRBC通過一個不依賴穿孔素而是由GzmB調(diào)節(jié)的機(jī)制導(dǎo)致衰亡。
結(jié)果表明，NK細(xì)胞對iRBC的活性可以被TKD肽特異性增強，也可以因為Hsp70被抑制而降低到基礎(chǔ)水平。因此，TKD可以作為一個革新性的免疫刺激物。
Plasmodium falciparum-Infected Erythrocytes Induce Granzyme B by NK Cells through Expression of Host-Hsp70
Evelyn Bttger, Gabriele Multhoff, Jürgen F. J. Kun, Meral Esen
In the early immune response to Plasmodium falciparum-infected erythrocytes （iRBC）, Natural Killer （NK） cells are activated, which suggests an important role in innate anti-parasitic immunity. However, it is not well understood whether NK cells directly recognize iRBC or whether stimulation of NK cells depends mainly on activating signals from accessory cells through cell-to-cell contact or soluble factors.In the present study, we investigated the influence of membrane-bound host Heat shock protein （Hsp） 70 in triggering cytotoxicity of NK cells from malaria-nave donors or the cell line NK92 against iRBC. Hsp70 and HLA-E membrane expression on iRBC and potential activatory NK cell receptors （NKG2C, CD94） were assessed by flow cytometry and immunoblot. Upon contact with iRBC, Granzyme B （GzmB） production and release was initiated by unstimulated and Hsp70-peptide （TKD） pre-stimulated NK cells, as determined by Western blot, RT-PCR and ELISPOT analysis. Eryptosis of iRBC was determined by Annexin V-staining.Our results suggest that presence of Hsp70 and absence of HLA-E on the membrane of iRBC prompt the infected host cells to become targets for NK cell-mediated cytotoxicity, as evidenced by impaired parasite development. Contact of iRBC with NK cells induced release of GzmB. We propose that following GzmB uptake, iRBC undergo eryptosis via a perforin-independent, GzmB-mediated mechanism.Since NK activity toward iRBC could be specifically enhanced by TKD peptide and abrogated to baseline levels by blocking Hsp70 exposure, we propose TKD as an innovative immunostimulatory agent to be tested as an adjunct to anti-parasitic treatments in vivo.
