王敬+雍康+吴有华+杨庆稳
摘要:乳腺内细菌感染可引起奶牛慢性型、临床型和亚临床型乳房炎。不同临床结果的出现,主要与细菌感染后激活乳腺免疫系统的方式不同有关。清晰地认识乳腺免疫系统激活和调节机制,对制定有效防治方案很有必要。综述了大肠杆菌性乳房炎和金黄色葡萄球菌性乳房炎的乳腺防御机制,并对二者的免疫反应进行了比较,旨在为奶牛乳房炎的临床防治提供参考依据。
关键词:奶牛;乳房炎;乳腺;细菌感染;免疫反应
中图分类号:S858.23;S857.26 文献标识码:A文章编号:0439-8114(2014)09-1989-04
Advances on Defensive Mechanisms of Dairy Cow Mammary Gland Infection
WANG Jing,YONG Kang,WU You-hua,YANG Qing-wen
(Department of Animal Science and Technology, Chongqing Three Gorges Vocational College, Chongqing 404155, China)
Abstract: Intramammary bacterial infections can be caused by bovine clinical type, chronic and subclinical mastitis. Different clinical results are mainly related with bacterial infection after activating the immune system in different ways of breast. It is necessary to well understand the activation and regulation mechanism of the breast immune system for developing effective prevention programs. This paper reviews breast defensive mechanisms of the Escherichia coli mastitis and Staphylococcus aureus mastitis and the immune response to the two bacteria.
Key words: cow; mastitis; mammary gland; bacterial infection; immune response
乳房炎是奶牛常见病和多发病,也是危害奶牛业最为严重的一种疾病。注射抗生素是国内外控制该病的主要手段,但细菌耐药性和药物残留等问题迫使人们寻找更加安全、有效的方法来防治乳房炎。通过营养调控、遗传选育等各种策略提高对乳房炎的抗性,降低乳房炎的发生率被认为是防治该病的长久之策,实现这一目标就需要对乳腺的防御机制进行深入了解[1]。
大部分乳腺感染是细菌突破了乳头管的生理屏障。细菌一旦进入乳池,就会大量生长繁殖,如果先天免疫反应未能及时建立,细菌就会定居在乳腺组织[2]。在分娩和泌乳早期,乳腺免疫功能下降,并且体细胞数(Somatic cell count,SCC)会在感染之前降低,这些因素均增加了乳房炎发生的风险。除了这些宿主因素外,细菌种类的不同也会影响乳腺的免疫反应,最终出现不同的临床结果[3]。
1乳腺的免疫反应机制
1.1病原识别和免疫反应的启动
一个快速而有效的先天免疫反应是基于对潜在病原体的早期识别[4]。先天免疫系统是非特异性的,当表面有特定的模式识别受体(Pattern recognition receptor,PRR)或在宿主细胞内结合特定的细菌时先天免疫系统被启动。当致病或非致病性微生物复制或降解时,这些不同的病原体所共有的保守结构就会释放[5]。
PRR在牛奶中的白细胞和乳腺上皮细胞表达[6]。Toll样受体(The toll-like receptor,TLR)群是哺乳动物中发现13种类型受体中最具特色的一种。TLRs的激活启动各种免疫调节剂的转录,随后核因子NF-kB迁移进入细胞核。另一个重要的病原受体CD14在乳腺嗜中性粒细胞和巨噬细胞中被发现,它结合脂多糖(Lipopolysaccharide,LPS)-蛋白质复合物并诱导肿瘤坏死因子TNF-α的合成和释放[5]。
1.2乳腺的免疫应答
病原体被识别后,血液中的白细胞高效转移到乳腺组织来抑制细菌生长[2]。具体来说,细菌感染后,嗜中性粒细胞被大量招募,通过超氧离子、次氯酸盐、过氧化氢和其他可溶性防御因素,如乳铁蛋白和防御素,可吞噬和杀死入侵的微生物[7]。
TNF-α既是乳腺免疫应答最早的起始中心,在急性大肠杆菌性乳房炎中,也是发热、内毒素休克发展的中心。白细胞介素IL-8、 受激活调节的正常T细胞表达和分泌因子(RANTES)等趋化因子是从血液中吸引白细胞的关键。补体、乳铁蛋白和溶菌酶也为炎症反应提供了便利[2,5]。
2不同病原体引发乳腺的免疫反应
虽然病原真菌能引起乳房炎,但最常见的是细菌。大肠杆菌(Escherichia coli)和金黄色葡萄球菌(Staphylococcus aureus)是引起乳房炎最常见的两种细菌[5]。
2.1大肠杆菌感染引起的乳房炎
急性大肠杆菌性乳房炎的特点是SCC快速和大量增加[8]。细胞壁LPS是这种形式乳房炎的发病核心[9]。宿主细胞通过与乳腺上皮细胞TLR-4的相互作用识别LPS[10]。LPS以剂量依赖性诱导SCC快速强烈增加,多种细胞因子如TNF-α,抗菌防御蛋白(如乳铁蛋白和溶菌酶)和脂质介质(如环氧合酶-2和5-脂氧合酶),也由该内毒素诱导[11,12]。
2.2金黄色葡萄球菌感染引起的乳房炎
金黄色葡萄球菌性乳房炎往往引起SCC的增加且持续时间较长,感染后可导致慢性和亚临床性乳房炎[8]。被感染的乳腺细胞中TNF-α mRNA增量表达[13]。
相反,革兰氏阴性菌脂多糖是主要免疫刺激分子,脂蛋白、肽聚糖和脂磷壁酸(Lipoteichoicacid,LTA)已被确定为革兰氏阳性菌细胞壁成分,它们均被TLR-2所识别[5]。Yang等[14]研究发现,金黄色葡萄球菌没有诱导乳腺上皮细胞的NF-kB级联,Bougarn等[15]研究证明参与NF-kB刺激的细胞因子被LTA激活。此外,乳腺上皮细胞TLR-2是由金黄色葡萄球菌[10]和LTA激活[6]。
在体外,LTA通过乳腺上皮细胞诱导TNF-α、IL-6和IL-8的表达[16],肽聚糖或其他革兰氏阳性细菌致病成分额外的刺激作用没有使这个表达增强。Rainard等[17]证实乳房内灌注LTA与体内感染金黄色葡萄球菌的免疫反应相比,牛奶中趋化因子和IL-1β增加,但TNF-α增加不明显。
2.3其他病原体引起的乳房炎
乳房链球菌(Streptococcus uberis)是引发乳房炎的常见病原菌,它诱导TNF-α、IL-1β和IL-8分泌进入乳汁[18]。有趣的是,一株从急性乳房炎中分离得到的金黄色葡萄球菌在体外诱导乳腺上皮细胞IL-8和IL-1β的表达量比慢性乳房炎中分离得到的菌株诱导分别高出2倍和4倍[19]。此外,支原体、铜绿假单胞菌(Pseudomonas aeruginosa)、黏质沙雷氏菌(Serratia marcescens)和肺炎克雷伯菌(Klebsiella pneumoniae)对牛乳房炎症反应的影响也有报道[20]。
3不同病原菌引发乳腺免疫反应的差异
3.1大肠杆菌和金黄色葡萄球菌引发乳腺免疫反应的差异
大肠杆菌性乳房炎发病急、病情重,经常导致奶牛死亡。相反,金黄色葡萄球菌性乳房炎往往呈慢性亚临床型。与大肠杆菌性乳房炎相比,金黄色葡萄球菌性乳房炎在牛奶中补体因子5a水平较低,TNF-α、IL-1β和IL-8的浓度没有增加,这些生物诱导不同的免疫反应最终表现在临床上的差异[8]。然而,在这些研究中,尽管感染量是一定的,金黄色葡萄球菌在乳房内的生长速率比大肠杆菌低得多。细菌生长速率的差异也可以解释为什么大肠杆菌感染比金黄色葡萄球菌感染乳腺中β-防御素、TLR-2和TLR-4产量多。即使使用多于大肠杆菌20倍剂量的金黄色葡萄球菌感染,结果依然如此[3]。
与大肠杆菌感染相比,葡萄球菌感染的细胞中IL-8水平更高[21]。在体外,大肠杆菌感染24 h后乳腺上皮细胞IL-1β、TNF-α和IL-8 mRNA比金黄色葡萄球菌表达量大[22]。然而,感染后3 h,金黄色葡萄球菌所诱导的这些细胞因子的表达量比大肠杆菌明显,但这种差异在感染10 h后消失。相同浓度下,热灭活的大肠杆菌和金黄色葡萄球菌刺激体外培养的乳腺上皮细胞,TNF-α、IL-1β、IL-6、IL-8、RANTES和C3的表达量比大肠杆菌刺激在体乳腺上皮细胞产生的多[10]。Strandberg等[6]用LTA诱导体外培养的乳腺上皮细胞时发现,细胞因子的表达量比LPS诱导产生的少。总的来说,这个标准是很重要的,可量化的参数如SCC被用于不同PAMPS乳腺免疫反应的比较研究[5]。
Wellnitz等[5]发现0.2 μg的LPS和20 μg的LTA分别接种到泌乳奶牛乳腺后诱导大约相同的SCC的增加,虽然诱导的细胞因子有变化,乳腺在LPS刺激下,牛奶中TNF-α、乳酸脱氢酶的浓度、TNF-α和IL-1 mRNA的表达量增加而LTA刺激并没有出现类似的变化,LPS也是牛奶中细胞IL-8和RANTES更强的诱导剂。对人类白细胞的研究还表明,LTA是TNF-α的一个较弱的诱导因子。综上所述,大肠杆菌和金黄色葡萄球菌激活乳腺免疫反应的重要差异影响了这些感染的临床表现[23](表1)。
乳腺感染易受到SCC的影响,低计数会增加乳房炎的风险和严重性[5]。Suriyasathaporn等[24]发现,低浓度的SCC会增加奶牛患严重大肠杆菌性乳房炎的风险,而高计数,动物则更易发生金黄色葡萄球菌性乳房炎。在感染乳房内SCC增加或降低的速度因病原菌而异:大肠杆菌可引起SCC快速、高剂量的增加,而金黄色葡萄球菌感染会使SCC在2~3 d内逐渐增加。Djabri等[25]计算不同的细菌感染乳腺SCC的平均值发现,大肠杆菌SCC的计数比金黄色葡萄球菌更高。
3.2其他细菌乳腺免疫反应的差异
与大肠杆菌和金黄色葡萄球菌相比,有关其他病原体的乳腺免疫反应的报道较少。Bannerman等[26]对比了导致奶牛临床型乳房炎发生的革兰阴性黏质沙雷氏菌和革兰阳性链球菌的乳腺反应发现,这两种细菌诱导牛奶中TNF-α、IL-1β水平提高。在体外,与灭活的金黄色葡萄球菌诱导相比,热灭活的链球菌没有触发乳腺上皮细胞类似的免疫反应,虽然这两种细菌都属于革兰氏阳性菌且均含有LTA[16]。
4展望
奶牛乳腺细菌感染可导致不同类型乳房炎的发生。这些差异是以不同的细菌在乳腺活化先天免疫反应为基础的。被感染乳腺中,细胞因子和SCC最终决定乳房炎的严重程度和发展速度,这有助于解释为什么大肠杆菌比金黄色葡萄球菌引起的乳房炎更急、更严重。目前对乳腺感染其他病原体,比如链球菌、支原体、真菌等所引发的防御机制研究还不深入,需要进一步探索。
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[15] BOUGARN S, CUNHA P, HARMACHE A, et al. Muramyl dipeptide synergizes with Staphylococcus aureus lipoteichoic acid to recruit neutrophils in the mammary gland and to stimulate mammary epithelial cells[J]. Clinical and Vaccine Immunology, 2010,17(11):1797-1809.
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[17] RAINARD P, FROMAGEAU A, CUNHA P, et al. Staphylococcus aureus lipoteichoic acid triggers inflammation in the lactating bovine mammary gland[J]. Veterinary Research,2008,39(5):52.
[18] RAMBEAUD M, ALMEIDA R A, PIGHETTI G M, et al. Dynamics of leukocytes and cytokines during experimentally-induced Streptococcus uberis mastitis[J]. Veterinary Immunology and Immunopathology,2003,96(3):193-205.
[19] WELLNITZ O, BERGER U, SCHAEREN W, et al. Mastitis severity induced by two Streptococcus uberis strains is reflected by the mammary immune response in vitro[J]. Schweiz Arch Tierheilkd,2012,154(8):317-323.
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[21] LEE J W, BANNERMAN D D, PAAPE M J, et al. Characterization of cytokine expression in milk somatic cells during intramammary infections with Escherichia coli or Staphylococcus aureus by real-time PCR[J]. Veterinary Research, 2006, 37(2):219-229.
[22] LAHOUASSA H, MOUSSAY E, RAINARD P, et al. Differential cytokine and chemokine responses of bovine mammary epithelial cells to Staphylococcus aureus and Escherichia coli[J]. Cytokine,2007,38(1):12-21.
[23] WENZ J R, FOX L K, MULLER F J, et al. Factors associated with concentrations of select cytokine and acute phase proteins in dairy cows with naturally occurring clinical mastitis[J]. Journal of Dairy Science, 2010,93(6):2458-2470.
[24] SURIYASATHAPORN W, SCHUKKEN Y H, NIELEN M, et al. Low somatic cell count:A risk factor for subsequent clinical mastitis in a dairy herd[J]. Journal of Dairy Science, 2000,83(6):1248-1255.
[25] DJABRI B, BAREILLE N, BEAUDEAU F, et al. Quarter milk somatic cell count in infected dairy cows: A meta-analysis[J]. Veterinary Research, 2002,33(4):335-357.
[26] BANNERMAN D D, PAAPE M J, GOFF J P, et al. Innate immune response to intramammary infection with Serratia marcescens and Streptococcus uberis[J]. Veterinary Research, 2004,35(6):681-700.
[10] GRIESBECK-ZILCH B, MEYER H H D, K?譈HN C H, et al. Staphylococcus aureus and Escherichia coli cause deviating expression profiles of cytokines and lactoferrin messenger ribonucleic acid in mammary epithelial cells[J]. Journal of Dairy Science,2008,91(6):2215-2224.
[11] WELLNITZ O,ARNOLD E T,BRUCKMAIER R M.Lipopolysa
-ccharide and lipoteichoic acid induce different immune responses in the bovine mammary gland [J]. J Dairy Sci, 2011,94(11):5405-5412.
[12] SCHMITZ S, PFAFFL M W, MEYER H H D, et al. Short-term changes of mRNA expression of various inflammatory factors and milk proteins in mammary tissue during LPS-induced mastitis[J]. Domestic Animal Endocrinology, 2004,26(2):111-126.
[13] ALLUWAIMI A M, LEUTENEGGER C M, FARVER T B, et al. The cytokine markers in Staphylococcus aureus mastitis of bovine mammary gland[J]. Journal of Veterinary Medicine, Series B: Infectious Diseases and Veterinary Public Health, 2003,50(3):105-111.
[14] YANG W, ZERBE H, PETZL W, et al. Bovine TLR2 and TLR4 properly transduce signals from Staphylococcus aureus and E. coli, but S. aureus fails to both activate NF-kappaB in mammary epithelial cells and to quickly induce TNF-alpha and interleukin-8(CXCL8) expression in the udder[J]. Molecular Immunology, 2008,45(5):1385-1397.
[15] BOUGARN S, CUNHA P, HARMACHE A, et al. Muramyl dipeptide synergizes with Staphylococcus aureus lipoteichoic acid to recruit neutrophils in the mammary gland and to stimulate mammary epithelial cells[J]. Clinical and Vaccine Immunology, 2010,17(11):1797-1809.
[16] WELLNITZ O, REITH P, HAAS S C, et al. Immune relevant gene expression of mammary epithelial cells and their influence on leukocyte chemotaxis in response to different mastitis pathogens[J]. Veterinarni Medicina,2006,51(4):125-132.
[17] RAINARD P, FROMAGEAU A, CUNHA P, et al. Staphylococcus aureus lipoteichoic acid triggers inflammation in the lactating bovine mammary gland[J]. Veterinary Research,2008,39(5):52.
[18] RAMBEAUD M, ALMEIDA R A, PIGHETTI G M, et al. Dynamics of leukocytes and cytokines during experimentally-induced Streptococcus uberis mastitis[J]. Veterinary Immunology and Immunopathology,2003,96(3):193-205.
[19] WELLNITZ O, BERGER U, SCHAEREN W, et al. Mastitis severity induced by two Streptococcus uberis strains is reflected by the mammary immune response in vitro[J]. Schweiz Arch Tierheilkd,2012,154(8):317-323.
[20] BANNERMAN D D. Pathogen-dependent induction of cytokines and other soluble inflammatory mediators during intramammary infection of dairy cows[J]. Journal of Animal Science, 2008,87(13):10-25.
[21] LEE J W, BANNERMAN D D, PAAPE M J, et al. Characterization of cytokine expression in milk somatic cells during intramammary infections with Escherichia coli or Staphylococcus aureus by real-time PCR[J]. Veterinary Research, 2006, 37(2):219-229.
[22] LAHOUASSA H, MOUSSAY E, RAINARD P, et al. Differential cytokine and chemokine responses of bovine mammary epithelial cells to Staphylococcus aureus and Escherichia coli[J]. Cytokine,2007,38(1):12-21.
[23] WENZ J R, FOX L K, MULLER F J, et al. Factors associated with concentrations of select cytokine and acute phase proteins in dairy cows with naturally occurring clinical mastitis[J]. Journal of Dairy Science, 2010,93(6):2458-2470.
[24] SURIYASATHAPORN W, SCHUKKEN Y H, NIELEN M, et al. Low somatic cell count:A risk factor for subsequent clinical mastitis in a dairy herd[J]. Journal of Dairy Science, 2000,83(6):1248-1255.
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