pcDNA3.1-Canstatin-3Flag真核表达载体的构建及转染HepG2细胞中的表达

目的:构建pcDNA3.1-Canstatin-3Flag载体并稳定转染肝癌HepG2细胞,检测canstatin在mRNA水平的表达。方法:胎盘中提取总RNA,RT-PCR法获得canstatinDNA,克隆至pcDNA3.1(-)载体中,并测序,重组质粒pcDNA3.1-Canstatin-3Flag转染肝癌HepG2细胞,G418筛选出稳定转染细胞,RT-PCR检测canstatin mRNA表达。结果:1.成功构建出pcDNA3.1-Canstatin-3Flag重组质粒;2.获得稳定转染pcDNA3.1-Canstatin-3Flag的肝癌HepG2细胞;3.发现转染后的肝癌HepG2细胞canstatin在mRNA水平比未转染细胞有明显的增强。结论:获得了稳定转染pcDNA3.1-Canstatin-3Flag的肝癌HepG2细胞,为后期canstatin在肝癌中的研究提供了支持。     

小鼠B7-H4基因的真核表达及其对淋巴细胞增殖的影响

目的构建小鼠B7-H4胞外段的真核表达载体,观察其在体外对淋巴细胞增殖的影响,为深入研究B7-H4在T细胞活化及移植排斥反应中的作用提供实验材料。方法提取小鼠肺、脾脏总RNA,RT-PCR反转录cDNA,以此为模板,扩增B7-H4胞外段基因,将其导入pGEM-T Easy载体,构建TA-mB7-H4质粒。用XBaI和HindIII双酶切后琼脂糖凝胶电泳分析和测序鉴定。将测序证实的mB7-H4酶切后装入MYC-HIS-EGFP-N荧光表达载体中,构建B7-H4-EGFP真核表达载体,转化JM109感受态细菌,提取重组质粒,酶切后琼脂糖凝胶电泳分析和测序鉴定。同时构建control-EGFP载体。应用脂质体法将重组质粒转染CHO细胞,经G418筛选,获得稳定表达B7-H4-EGFP的CHO细胞株,用MTT分析其分别对BALB/c小鼠、C57小鼠淋巴细胞和二者混合淋巴细胞增殖的影响。结果经测序证实,所克隆的小鼠B7-H4 cDNA和构建的重组质粒基因序列正确;转染的CHO细胞能稳定地表达跨膜型重组蛋白B7-H4;表达的B7-H4对淋巴细胞增殖具有明显抑制作用。结论成功构建了B7-H4真核表达系统,能表达有生物学活性的B7-H4分子,为进一步探讨B7-H4在T细胞活化和移植排斥反应中的作用奠定了基础。     

西方马脑炎核酸疫苗表达载体构建及免疫原性研究

为构建西方马脑炎病毒(western equine encephalomyelitis virus,WEEV)结构基因C-E3-E2-6k-E1重组真核表达载体,并研究其作为核酸疫苗的免疫原性.采用PCR方法扩增目的基因,酶切之后连接到pcDNA3.1上构建真核表达载体pcDNA3.1-C-E,用酶切和测序分析方法鉴定正确后,重组质粒被转染到293T细胞,经电镜检测和间接免疫荧光方法证明基因可以表达后,用该重组质粒免疫小鼠,免疫后检测实验组小鼠外周血中CD4+T细胞/CD8+T细胞比例和血清中细胞因子IL-2、IL-4及IFN-γ浓度,以上实验组各项免疫指标与对照组相比差异均显著(P＜ 0.05);ELISA方法检测实验组小鼠血清中WEEV的IgG抗体效价是1∶16.研究结果表明重组质粒pcDNA3.1-C-E可在细胞中获得瞬时表达,并且重组质粒作为核酸疫苗能够刺激小鼠产生免疫反应,具有较强免疫原性,为今后WEEV核酸疫苗研制奠定了良好基础.     

病毒负性调节因子在内皮细胞稳定表达细胞株的建立

建立HIV-1的调节基因Nef基因在内皮细胞稳定表达的细胞株ECV304-Nef,为研究Nef对血管内皮细胞生物学活性的影响奠定试验基础。构建真核表达载体pcDNA3.1(+)-Nef,将其质粒和pcDNA3.1(+)质粒(阴性对照)分别转染血管内皮细胞ECV304,G418筛选。通过RT-PCR检测NefmRNA在细胞中的表达;细胞免疫荧光法检测Nef蛋白的表达及定位;Western blotting检测Nef蛋白的特异性表达,获得稳定表达的细胞株。构建的重组质粒pcDNA3.1(+)-Nef经BamHI和EcoRI双酶切鉴定,得到的片段大小与理论值相符,分别为载体的5400bp和目的基因的621bp。测序结果显示碱基序列与GenBank(登录号:K03455)序列相同。转染细胞经G418筛选后获得稳定表达Nef的ECV304细胞株,RT-PCR显示转染pcD-NA3.1(+)-Nef质粒的ECV304细胞出现621bp条带,对照组无目的条带出现;荧光显微镜下观察转染pcDNA3.1(+)-Nef质粒的ECV304细胞表达的Nef蛋白主要定位于细胞质中。Western blotting结果显示,转染pcDNA3.1(+)-Nef质粒的ECV304细胞约27kD处检测到目的条带,表明pcDNA3.1(+)-Nef表达正确。     

hTEM8-N-RFP融合蛋白真核表达载体的构建及在HEK293F细胞中表达

目的:构建人肿瘤内皮标志物8(hTEM8)胞外区(N端)与红色荧光蛋白(RFP)融合表达载体并在HEK293F细胞中表达,为进一步研究hTEM8的相互作用蛋白及其在肿瘤血管形成过程中的机制奠定实验基础。方法:以质粒pDsRed-Express-C1和重组质粒pcDNA3.1(+)-hTEM8/Fc为模板,PCR扩增RFP和hTEM8-N基因片段,先后插入真核表达载体pcDNA3.1(+)中,构建重组表达载体pcDNA3.1(+)-hTEM8-N-RFP,转染HEK293F细胞,通过荧光显微镜观察融合蛋白在转染细胞中的的表达,并用G418对转染的细胞进行加压筛选,Western blot检测hTEM8-N-RFP融合蛋白在转染细胞中的表达。结果:DNA测序、酶切鉴定的结果显示,表达载体pcDNA3.1(+)-hTEM8-N-RFP构建成功,且序列正确。转染后经荧光显微镜观察到HEK293F细胞中有红色荧光,经加压筛选单克隆后,在荧光显微镜下观察到稳定表达红色荧光的细胞株,Western blot检测到融合蛋白hTEM8-N-RFP在真核细胞HEK293F中获得表达。结论:成功构建了pcDNA3.1(+)-hTEM8-N-RFP真核表达载体,并在HEK293F细胞中表达,为后期研究hTEM8的相互作用蛋白和其生理功能奠定了良好的基础。     

怀化医学高等专科学校医学形态学实验中心

目的:探讨抑癌基因p16对肝癌细胞生长的抑制作用及其机制。方法:将p16 cDNA亚克隆至pcDNA3.1真核表达载体上,并经脂质体介导转染至人肝癌细胞株SMMC-7721。用MTT法和Western blot分析转染细胞的生长情况。结果:成功构建重组表达质粒pcDNA3.1-p16,转染pcDNA3.1-p16的SMMC-7721细胞生长速度受到明显抑制;转染后有外源p16蛋白的表达,且伴随Bax上调,Bcl-2和cIAP2的下调。结论:重组pcDNA3.1-p16质粒能在人肝癌细胞SMMC-7721内表达,且能抑制SMMC-7721的生长,其机理与诱导肿瘤细胞凋亡相关。     

稳定表达乙型流感病毒血凝素蛋白细胞的构建

获得稳定表达乙型流感病毒血凝素蛋白的HEK-293-HA细胞株。采用RT-PCR方法扩增乙型流感病毒的HA基因,将其克隆至真核表达载体pcDNA3.1(+)上,构建pcDNA3.1(+)-HA重组质粒。将已鉴定正确的pcDNA3.1(+)-HA质粒与辅助质粒PLV-EF1a-EGFP(2A)Puro通过脂质体介导法共转染HEK293细胞,经嘌呤霉素筛选后得到重组细胞株HEK293-HA,通过流式细胞术法(FCM)来检测细胞中HA的表达情况。所得到的重组细胞经扩大培养后再连续培养15代,采用间接免疫荧光法(IFA)和蛋白免疫印迹法(WB)来检测HA蛋白表达的稳定性。结果表明重组质粒pcDNA3.1(+)-HA经双酶切及测序鉴定正确;共获得3株高表达阳性细胞株。细胞扩大培养后连续传代培养15代后进行Western blot和IFA检测结果表明,重组细胞的HA蛋白得到稳定的表达。已成功获得了能稳定表达乙型流感病毒血凝素蛋白的HEK-293-HA细胞,可为乙型流感病毒HA蛋白的进一步研究提供良好的基础。     

小鼠EVL 基因的克隆和真核表达载体的构建

目的:构建小鼠EVL(Ena/VASP like)基因的真核表达载体,为深入研究EVL的功能奠定基础.方法:采用PCR方法,从小鼠cDNA文库中,扩增出1245bp的EVL编码区片段,经电泳、胶回收后连接入pMD- 18T载体中,测序鉴定正确.用BamHI和HincⅡ双酶切,定向克隆EVL编码区片段到真核表达载体pcDNA3.1中,用限制性内切酶酶切鉴定重组质粒正确后.将重组质粒转染入HELA细胞中,以RT-PCR检测EVL的mRNA的表达,以Western Blot检测EVL蛋白的表达.结果:酶切鉴定结果显示小鼠EVL编码区基因被成功克隆入真核表达载体pcDNA3.1中;RT-PCR和Western Blot结果以及免疫荧光染色显示Hela细胞中有EVL的mRNA和蛋白的表达.结论:成功获得pcDNA3.1 -EVL的真核表达载体,为进一步深入研究EVL蛋白的功能奠定了基础.     

嗜肺军团菌mip/ctxB融合基因体外表达与动物免疫试验的免疫原性研究

ＰＣＲ扩增嗜肺军团菌mip基因和霍乱弧菌ctxB基因,克隆入载体pcDNA3.1(+),重组子经限制性酶切分析、PCR、序列分析鉴定正确后,命名为pcDNA3.1-mip/ctxB.脂质体法将重组质粒pcDNA3.1-mip、pcDNA3.1-mip/ctxB转染NIH3T3细胞,用免疫荧光法和蛋白质印迹鉴定瞬时表达和稳定表达产物,结果发现：重组质粒成功转入细胞并获得短暂表达,稳定转染细胞分别在24 ku和35 ku处检测到阳性杂交信号.将pcDNA3.1-mip、pcDNA3.1-mip/ctxB作为DNA疫苗免疫BALB/c小鼠,检测免疫小鼠体内抗原特异性抗体水平、脾淋巴细胞增殖活性、IFN-γ产生水平、细胞毒性T淋巴细胞(CTL)杀伤活性等体液免疫和细胞免疫反应的指标,评价疫苗的免疫原性.结果发现：各实验组均检测到免疫原性,pcDNA3.1-mip/ctxB免疫组的免疫原性高于pcDNA3.1-mip免疫组,有显著性差异(P＜0.01).研究结果为mip/ctxB融合基因DNA疫苗的研制提供了初步的实验依据.     

转基因小鼠模型的载体构建和在人肝癌HepG2细胞中的表达

目的:构建转基因小鼠模型的载体并检测在人肝癌HepG2中的表达效果.方法:将n-3多不饱和脂肪酸脱氢酶基因fat-1插入到真核表达载体(pcDNA3.1(+)myc-HisA)中,构建重组表达载体pcDNA3.1(+)myc-His A-fat-1,用脂质体介导的方法转染到人肝癌HepG2细胞中,RT-PCR检测fat-l基因的表达,MTT法分析fat-l基因对HepG2细胞增殖的影响,气相色谱分析检测fat-l基因对HepG2细胞n-6/n-3多聚不饱和脂肪酸(PUFAs)比例的影响.结果:成功地构建了真核表达裁体peDNA3.1(+)myc-HisA-fat-1,并能在HepG2细胞内有效异源表达.48h后可检测到fat-l mRMA的条带.与对照细胞相比,fat-l基因有效地抑制了人肝癌细胞HepG2细胞的增殖(70%,p＜0.01),降低了n-6/n-3 PUFAs比例.结论:pcDNA3.1(+)myc-His A-fat-l重组载体构建成功并能在肝癌细胞中有效的表达,可以作为下一步转基因小鼠的合适载体.     

The role of CTLA-4 in regulating Th2 differentiation.

To examine the role of CTLA-4 in Th cell differentiation, we used two newly generated CTLA-4-deficient (CTLA-4-/-) mouse strains: DO11. 10 CTLA-4-/- mice carrying a class II restricted transgenic TCR specific for OVA, and mice lacking CTLA-4, B7.1 and B7.2 (CTLA-4-/- B7.1/B7.2-/- ). When purified naive CD4+ DO11.10 T cells from CTLA-4-/- and wild-type mice were primed and restimulated in vitro with peptide Ag, CTLA-4-/- DO11.10 T cells developed into Th2 cells, whereas wild-type DO11.10 T cells developed into Th1 cells. Similarly, when CTLA-4-/- CD4+ T cells from mice lacking CTLA-4, B7. 1, and B7.2 were stimulated in vitro with anti-CD3 Ab and wild-type APC, these CTLA-4-/- CD4+ T cells produced IL-4 even during the primary stimulation, whereas CD4+ cells from B7.1/B7.2-/- mice did not produce IL-4. Upon secondary stimulation, CD4+ T cells from CTLA-4-/- B7.1/B7.2-/- mice secreted high levels of IL-4, whereas CD4+ T cells from B7.1/B7.2-/- mice produced IFN-gamma. In contrast to the effects on CD4+ Th differentiation, the absence of CTLA-4 resulted in only a modest effect on T cell proliferation, and increased proliferation of CTLA-4-/- CD4+ T cells was seen only during secondary stimulation in vitro. Administration of a stimulatory anti-CD28 Ab in vivo induced IL-4 production in CTLA-4-/- B7.1/B7.2-/- but not wild-type mice. These studies demonstrate that CTLA-4 is a critical and potent inhibitor of Th2 differentiation. Thus, the B7-CD28/CTLA-4 pathway plays a critical role in regulating Th2 differentiation in two ways: CD28 promotes Th2 differentiation while CTLA-4 limits Th2 differentiation.     

Correct identification of the chloroplastic protoporphyrinogen IX oxidase N-terminus places the biochemical data in frame

CTLA-4 gene constructs were designed to express CTLA-4 exclusively in the endoplasmic reticulum (ER). Four different CTLA-4 gene constructs were transfected into HEK 293 (human embryonic kidney) and A20 (Balb/c mouse B lymphoma) cells. All constructs contained an ER retention signal and coded for CTLA-4 expression in the ER. One of the constructs, which contained the membrane part of CTLA-4, coded for an expression both on the cell surface and in the ER. Three of the expressed CTLA-4 types (including the ER-membrane-expressed form) caused a reduced surface expression of B7 in the A20 cells. Only constructs which allow dimerization of CTLA-4 showed this effect. It is assumed that intracellular CTLA-4 bound B7 and inhibited therefore the transport of B7 to the surface. The binding obviously caused also an enhanced degradation of the complexes because both proteins showed a low concentration in the transfected cell lines. CTLA-4-transfected and B7-reduced A20 cells showed a diminished costimulating activity upon T cells. This was demonstrated by a reduced proliferation of T cells from ovalbumin-immunized Balb/c mice, incubated with ovalbumin peptide-primed CTLA-4-transfected A20 cells.     

Low CD86 expression in the nonobese diabetic mouse results in the impairment of both T cell activation and CTLA-4 up-regulation

The nonobese diabetic (NOD) mouse spontaneously develops autoimmune insulin-dependent diabetes mellitus and serves as a model for human type I diabetes. NOD spleen cells proliferate to a lesser extent than those from C57BL/6 and BALB/c mice in response to anti-CD3. To investigate the cause of this reduced T cell proliferation, costimulatory molecule expression was investigated. It was found that NOD macrophages, dendritic cells, and T cells, but not B cells, expressed lower basal levels of CD86, but not CD80, CD28, or CD40, compared with C57BL/6 and BALB/c. This low CD86 expression was not dependent on the MHC haplotype or on diabetes development since the NOD-related, diabetes-free mouse strains NON (H-2nb1) and NOR (H-2g7) exhibited similar low levels of CD86 expression and proliferation. Furthermore, following activation, the relative up-regulation of CTLA-4, as compared with CD28, was more pronounced on C57BL/6 and BALB/c T cells as shown by an increased CTLA-4/CD28 ratio. This activation-induced increase in the CTLA-4/CD28 ratio was markedly reduced on NOD T cells compared with the other two strains. The low CD86 expression in NOD mice may account for the reduced increase in both proliferation and the CTLA-4/CD28 ratio, since reducing CD86 expression in C57BL/6 and BALB/c cultures to NOD levels significantly reduces the proliferation and the CTLA-4/CD28 ratio. Therefore, we propose that a low level of CD86 expression in the NOD mouse contributes to a defective regulation of autoreactive T cells by preventing the full activation of T cells and therefore the up-regulation of CTLA-4.     

Ligation of cytotoxic T lymphocyte antigen-4 to T cell receptor inhibits T cell activation and directs differentiation into Foxp3+ regulatory T cells

Cross-linking of ligand-engaged cytotoxic T lymphocyte antigen-4 (CTLA-4) to the T cell receptor (TCR) during the early phase of T cell activation attenuates TCR signaling, leading to T cell inhibition. To promote this event, a bispecific fusion protein comprising a mutant mouse CD80 (CD80w88a) and lymphocyte activation antigen-3 was engineered to concurrently engage CTLA-4 and cross-link it to the TCR. Cross-linking is expected to be attained via ligation of CTLA-4 first to MHCII and then indirectly to the TCR, generating a CTLA-4-MHCII-TCR trimolecular complex that forms between T cells and antigen-presenting cells during T cell activation. Treating T cells with this bispecific fusion protein inhibited T cell activation. In addition, it induced the production of IL-10 and TGF-β and attenuated AKT and mTOR signaling. Intriguingly, treatment with the bispecific fusion protein also directed early T cell differentiation into Foxp3-positive regulatory T cells (Tregs). This process was dependent on the endogenous production of TGF-β. Thus, bispecific fusion proteins that engage CTLA-4 and co-ligate it to the TCR during the early phase of T cell activation can negatively regulate the T cell response. Bispecific biologics with such dual functions may therefore represent a novel class of therapeutics for immune modulation. These findings presented here also reveal a potential new role for CTLA-4 in Treg differentiation.     

Enhanced engagement of CTLA-4 induces antigen-specific CD4+CD25+Foxp3+ and CD4+CD25- TGF-beta 1+ adaptive regulatory T cells

CTLA-4 is a critical negative regulator of T cell response and is instrumental in maintaining immunological tolerance. In this article, we report that enhanced selective engagement of CTLA-4 on T cells by Ag-presenting dendritic cells resulted in the induction of Ag-specific CD4(+)CD25(+)Foxp3(+) and CD4(+)CD25(-)TGF-beta1(+) adaptive Tregs. These cells were CD62L(low) and hyporesponsive to stimulation with cognate Ag but demonstrated a superior ability to suppress Ag-specific effector T cell response compared with their CD62L(high) counterparts. Importantly, treatment of mice with autoimmune thyroiditis using mouse thyroglobulin (mTg)-pulsed anti-CTLA-4 agonistic Ab-coated DCs, which results in a dominant engagement of CTLA-4 upon self-Ag presentation, not only suppressed thyroiditis but also prevented reemergence of the disease upon rechallenge with mTg. Further, the disease suppression was associated with significantly reduced mTg-specific T cell and Ab responses. Collectively, our results showed an important role for selective CTLA-4 signaling in the induction of adaptive Tregs and suggested that approaches that allow dominant CTLA-4 engagement concomitant with Ag-specific TCR ligation can be used for targeted therapy.     

Exosomes-delivered PD-L1 siRNA and CTLA-4 siRNA protect against growth and tumor immune escape in colorectal cancer

ObjectiveThis study aims to dissect impacts of exosomes-delivered PD-L1 and CTLA-4 siRNAs on colorectal cancer (CRC) progression and immune responses.MethodsExosomes containing PD-L1 siRNA and CTLA-4 siRNA were prepared and utilized to treat CRC cells to evaluate their effects. A tumor-bearing mouse model was established for verification.ResultsExosomes containing PD-L1 siRNA and CTLA-4 siRNA repressed malignant features of CRC cells and restrained tumor growth and activated tumor immune responses in vivo. Co-culture of CRC cells treated with exosomes containing PD-L1 siRNA and CTLA-4 siRNA with human CD8⁺ T cells increased the percentage of CD8⁺ T cells, decreased the apoptotic rate of CD8⁺ T cells, elevated IL-2, IFN-γ, and TNF-α expression in cell supernatants, reduced adherent density of CRC cells, augmented the positive rate of CRC cells, and subdued tumor immune escape.ConclusionExosomes containing PD-L1 siRNA and CTLA-4 siRNA suppressed CRC progression and enhanced tumor immune responses.     

Absence of CTLA-4 lowers the activation threshold of primed CD8+ TCR-transgenic T cells: lack of correlation with Src homology domain 2-containing protein tyrosine phosphatase

To examine the role of CTLA-4 in controlling Ag-specific CD8(+) T cell activation, TCR-transgenic/CTLA-4 wild-type or -deficient mice were generated in a recombination-activating gene 2-deficient background. Naive T cells from these mice responded comparably whether or not CTLA-4 was expressed. In contrast, primed T cells responded more vigorously if they lacked CTLA-4 expression. We took advantage of the difference between naive and primed T cell responses to approach the mechanism of CTLA-4 function. Single-cell analyses demonstrated that a greater fraction of CTLA-4-deficient cells responded to a fixed dose of Ag compared with CTLA-4-expressing cells, whereas the magnitude of response per cell was comparable. A shift in the dose-response curve to APCs was also observed such that fewer APCs were required to activate CTLA-4-deficient T cells to produce intracellular IFN-gamma and to proliferate. These results suggest that CTLA-4 controls the threshold of productive TCR signaling. Biochemical analysis comparing stimulated naive and primed TCR-transgenic cells revealed no obvious differences in expression of total CTLA-4, tyrosine-phosphorylated CTLA-4, and associated Src homology domain 2-containing protein tyrosine phosphatase. Thus, the biochemical mechanism explaining the differential inhibitory effect of CTLA-4 on naive and primed CD8(+) T cells remains unclear.     

TGF-beta requires CTLA-4 early after T cell activation to induce FoxP3 and generate adaptive CD4+CD25+ regulatory cells

Although positive CD28 costimulation is needed for the generation of natural CD4+CD25+ regulatory T cells, we report that negative CTLA-4 costimulation is necessary for generating phenotypically and functionally similar adaptive CD4+CD25+ suppressor cells. TGF-beta could not induce CD4+CD25- cells from CTLA-4(-/-) mice to express normal levels of FoxP3 or to develop suppressor activity. Moreover, blockade of CTLA-4 following activation of wild-type CD4+ cells abolished the ability of TGF-beta to induce FoxP3-expressing mouse suppressor cells. TGF-beta accelerated expression of CTLA-4, and time course studies suggested that CTLA-4 ligation of CD80 shortly after T cell activation enables TGF-beta to induce CD4+CD25- cells to express FoxP3 and develop suppressor activity. TGF-beta also enhanced CD4+ cell expression of CD80. Thus, CTLA-4 has an essential role in the generation of acquired CD4+CD25+ suppressor cells in addition to its other inhibitory effects. Although natural CD4+CD25+ cells develop normally in CTLA-4(-/-) mice, the lack of TGF-beta-induced, peripheral CD4+CD25+ suppressor cells in these mice may contribute to their rapid demise.     

Cutting edge: cell-extrinsic immune regulation by CTLA-4 expressed on conventional T cells

The CTLA-4 pathway is a key regulator of T cell activation and a critical failsafe against autoimmunity. Although early models postulated that CTLA-4 transduced a negative signal, in vivo evidence suggests that CTLA-4 functions in a cell-extrinsic manner. That multiple cell-intrinsic mechanisms have been attributed to CTLA-4, yet its function in vivo appears to be cell-extrinsic, has been an ongoing paradox in the field. Although CTLA-4 expressed on conventional T cells (Tconv) can mediate inhibitory function, it is unclear why this fails to manifest as an intrinsic effect. In this study, we show that Tconv-expressed CTLA-4 can function in a cell-extrinsic manner in vivo. CTLA-4(+/+) T cells, from DO11/rag(-/-) mice that lack regulatory T cells, were able to regulate the response of CTLA-4(-/-) T cells in cotransfer experiments. This observation provides a potential resolution to the above paradox and suggests CTLA-4 function on both Tconv and regulatory T cells can be achieved through cell-extrinsic mechanisms.