HIV Nef, paxillin, and Pak1/2 regulate activation and secretion of TACE/ADAM10 proteases

The HIV Nef protein recruits the polycomb protein Eed and mimics an integrin receptor signal for reasons that are not entirely clear. Here we demonstrate that Nef and Eed complex with the integrin effector paxillin to recruit and activate TNFα converting enzyme (TACE alias ADAM 17) and its close relative ADAM10. The activated proteases cleaved proTNFα and were shuttled into extracellular vesicles (EVs). Peripheral blood mononuclear cells that ingested these EVs released TNFα. Analyzing the mechanism, we found that Pak2, an established host cell effector of Nef, phosphorylated paxillin on Ser272/274 to induce TACE-paxillin association and shuttling into EVs via lipid rafts. Conversely, Pak1 phosphorylated paxillin on Ser258, which inhibited TACE association and lipid raft transfer. Interestingly, melanoma cells used an identical mechanism to shuttle predominantly ADAM10 into EVs. We conclude that HIV-1 and cancer cells exploit a paxillin/integrin-controlled mechanism to release TACE/ADAM10-containing vesicles, ensuring better proliferation/growth conditions in their microenvironment.

IL-21 reduces production of pro-inflammatory cytokines by activated CD8 T cells (44.26)

Background: Chronic HIV infection is characterized by systemic immune activation (IA), accumulation of T cells with a skewed terminally-differentiated profile and a progressive loss of IL-21 production. Experiments described below address the hypothesis that IL-21 prevents IA in healthy individuals. Methods: Purified CD8 T cells from healthy donors were activated for 48 hrs. by antiCD3+antiCD28 in the presence of IL-21 and, respectively, its absence. Cytokine synthesis and secretion were evaluated by real time PCR and ELISA. Results: CD8 T cells TCR-stimulated in the presence of IL-21 produced less pro-inflammatory cytokines TNF alpha and IL-12p70, compared to CTL stimulated in the absence of exogenous cytokine. Moreover, IL-21 induced significantly higher amounts of the anti-inflammatory cytokine IL-10 and lower levels of IL-2. Conclusions: IL-21 could act as a favorable immunomodulatory agent in the context of chronic infections such as HIV because: i) it limits the production of pro-inflammatory mediators by CD8 T cells; and ii) it induces the anti-inflammatory cytokine IL-10.

High affinity T cell receptor gives advantage to the homeostatic proliferation of naive CD8 T cells only under competitive pressure (126.28)

The impact of T cell receptor (TCR) affinity during homeostatic proliferation has been demonstrated using naive CD8+ T cells with high and low affinity TCR derived from different precursors. In this study, we investigated the impact of TCR affinity using cells derived from the same precursor and selected under the same conditions, namely Pmel TCR transgenic (vβ13) naive CD8+ T cells, which respond to both high (hgp100) and low (mgp100) affinity peptides. Contrary to previous reports, we found that exposure of naive Pmel cells to either high or low-affinity peptide resulted in similar levels of homeostatic proliferation. However, in the presence of OT-1 cells and their cognate peptide (SINFKL), the levels of homeostatic proliferation of Pmel cells were higher after exposure to hgp100 than to mgp100 (1.85 vs 1.25 on day 8, 2.06 vs 1.20 on day 11, 2.57 vs 1.30 on day 15, 2.35 vs 0.89 on day 22). Interestingly, expression levels of memory markers (CD44lo to CD44hi, CD122lo to CD122hi and CD62Lhi to CD62Lhi) were similar under both circumstances. Overall, these results suggest that the difference seen in the effect of high-affinity TCR compared with low-affinity TCR on CD8+ cell expansion during homeostatic proliferation occurs only in the presence of competitive pressure. This difference may be balanced by a higher availability of low-affinity antigen, and thus may have important implications for vaccine design in the post-transplant setting.

Interleukin (IL)-21, but not IL-2, induces antiviral activity and costimulatory molecules in CD8 T cells without promoting HIV replication (42.15)

Background: Chronic HIV infection is marked by defective cytotoxic T cell (CTL) effector function, skewed terminally-differentiated profile, and reduced expression of the costimulatory molecules CD27 and CD28. Incubation ex vivo of CTL from HIV infected persons with IL-21 augments cytotoxic effector molecules and function. In this study we analyzed IL-21 effects on CTL as compared to IL-2. Methods: Purified CD8 T cells from healthy donors were pre-activated with αCD3+αCD28 and cultured with IL-21 or IL-2 for 6 days. Phenotype and effector function were evaluated by flow cytometry, gene expression by real-time PCR, effect on HIV replication with the reporter cell line TZM-bl. Results: IL-21 induced higher CTL degranulation in response to TCR stimulation, as compared to IL-2. Co-culture of IL-21-treated CTL with autologous HIV infected CD4 T cells reduced HIV replication. Importantly, IL-21 did not induce viral replication when added to HIV infected CD4 T cells. Furthermore, TCR-activated CTL upreglated CD27 and CD28 and acquired a central memory phenotype when incubated with IL-21, but not with IL-2. IL-21 also selectively induced Bcl6, key transcription factor for memory T cells. Conclusions: Since IL-21 enhances CTL antiviral activity, promotes a central memory phenotype and induces costimulatory molecules, this cytokine has an appealing immunomodulatory profile and merits further consideration for cytokine-based therapies, alone or in combination with antiretroviral drugs.

Murine neonatal CD4+ cells are poised for rapid Th2 effector-like function

Murine neonates typically mount Th2-biased immune responses. This entails a cell-intrinsic component whose molecular basis is unknown. We found that neonatal CD4(+) T cells are uniquely poised for rapid Th2 function. Within 24 h of activation, neonatal CD4(+) cells made high levels of IL-4 and IL-13 mRNA and protein. The rapid high-level IL-4 production arose from a small subpopulation of cells, did not require cell cycle entry, and was unaffected by pharmacologic DNA demethylation. CpG methylation analyses in resting neonatal cells revealed pre-existing hypomethylation at a key Th2 cytokine regulatory region, termed conserved noncoding sequence 1 (CNS-1). Robust Th2 function and increased CNS-1 demethylation was a stable property that persisted in neonatal Th2 effectors. The transcription factor STAT6 was not required for CNS-1 demethylation and this state was already established in neonatal CD4 single-positive thymocytes. CNS-1 demethylation levels were much greater in IL-4-expressing CD4 single-positive thymocytes compared with unactivated cells. Together, these results indicate that neonatal CD4+ T cells possess distinct qualities that could predispose them toward rapid, effector-like Th2 function.

Epigenetic patterns at the neonatal Th2 cytokine locus are developmentally regulated (85.2)

Murine neonatal immune responses are commonly Th2 biased, characterized by rapid and high level Th2 cytokine production. These Th2-biased responses entail cell-intrinsic properties whose molecular basis is now beginning to become unveiled. We have recently found that the Th2 regulatory region CNS-1 is nearly 25% more demethylated in naïve neonatal CD4+ lymph node T cells and in CD4+ single positive (SP) thymocytes than in the same adult populations. Importantly, IL-4-expressing neonatal thymocytes exhibit substantially greater levels of hypomethylation at CNS-1 compared to unactivated cells. These results suggest that a hypomethylated state at CNS-1 is permissive for IL-4 expression in neonatal cells. Additional studies have revealed that CNS-1 exists in this highly demethylated state in freshly isolated fetal (d15 gestation) thymocytes, which are predominantly double negative cells and are the precursors of CD4+ SP thymocytes found in the neonate. These new findings suggest that epigenetic patterns at the neonatal Th2 cytokine locus are established within the fetal thymus, or even earlier, possibly in fetal liver precursor cells. Together, these observations lead to the intriguing idea that epigenetic modifications may be used as a developmental strategy within the CD4+ T helper cell lineage to achieve differential expression of cytokine genes at different times of life.

This work was supported by NIAID grant number R01 AI44923-02 (B.A.).

DNA methylation and chromatin structure regulate T cell perforin gene expression

Perforin is a cytotoxic effector molecule expressed in NK cells and a subset of T cells. The mechanisms regulating its expression are incompletely understood. We observed that DNA methylation inhibition could increase perforin expression in T cells, so we examined the methylation pattern and chromatin structure of the human perforin promoter and upstream enhancer in primary CD4(+) and CD8(+) T cells as well as in an NK cell line that expresses perforin, compared with fibroblasts, which do not express perforin. The entire region was nearly completely unmethylated in the NK cell line and largely methylated in fibroblasts. In contrast, only the core promoter was constitutively unmethylated in primary CD4(+) and CD8(+) cells, and expression was associated with hypomethylation of an area residing between the upstream enhancer at -1 kb and the distal promoter at -0.3 kb. Treating T cells with the DNA methyltransferase inhibitor 5-azacytidine selectively demethylated this area and increased perforin expression. Selective methylation of this region suppressed promoter function in transfection assays. Finally, perforin expression and hypomethylation were associated with localized sensitivity of the 5' flank to DNase I digestion, indicating an accessible configuration. These results indicate that DNA methylation and chromatin structure participate in the regulation of perforin expression in T cells.

Perforin mRNA in primary peritoneal exudate cytotoxic T lymphocytes

Considerable evidence indicates that cloned CTL cell lines kill target cells by releasing toxic granules that contain a cytolytic protein, called perforin, and several serine esterases (granzymes A to F). However, primary CTL, such as the highly cytolytic peritoneal exudate lymphocyte (PEL) cell population, have been found by a hemolytic assay to have no perforin, or perhaps only borderline levels of that protein, suggesting that these cells use a different lytic mechanism. To determine whether or not primary CTL express the perforin gene, we have here compared mRNA from PEL CTL and from a cloned CTL cell line, 2C, by Northern blot analysis using a perforin cDNA probe. CD8+ PEL CTL contain approximately 30% of the amount of perforin message present in 2C. Moreover, depletion of CD8+ T cells from the total peritoneal exudate cell population removes both cytolytic activity and perforin message. We have previously shown that PEL CTL elicit the same changes in target cells as cloned CTL cell lines and are resistant to lysis by the toxic granules purified from these cells lines. Taken together these results are consistent with the view that primary CTL, as well as long term cloned CTL cell lines, exercise their cytolytic activity by means of perforin.

Perforin mRNA in primary peritoneal exudate cytotoxic T lymphocytes

Considerable evidence indicates that cloned CTL cell lines kill target cells by releasing toxic granules that contain a cytolytic protein, called perforin, and several serine esterases (granzymes A to F). However, primary CTL, such as the highly cytolytic peritoneal exudate lymphocyte (PEL) cell population, have been found by a hemolytic assay to have no perforin, or perhaps only borderline levels of that protein, suggesting that these cells use a different lytic mechanism. To determine whether or not primary CTL express the perforin gene, we have here compared mRNA from PEL CTL and from a cloned CTL cell line, 2C, by Northern blot analysis using a perforin cDNA probe. CD8+ PEL CTL contain approximately 30% of the amount of perforin message present in 2C. Moreover, depletion of CD8+ T cells from the total peritoneal exudate cell population removes both cytolytic activity and perforin message. We have previously shown that PEL CTL elicit the same changes in target cells as cloned CTL cell lines and are resistant to lysis by the toxic granules purified from these cells lines. Taken together these results are consistent with the view that primary CTL, as well as long term cloned CTL cell lines, exercise their cytolytic activity by means of perforin.

Cloning, analysis, and expression of murine perforin 1 cDNA, a component of cytolytic T-cell granules with homology to complement component C9

The nucleotide sequence coding for the cytotoxic T-lymphocyte (CTL) protein perforin 1 (P1) has been determined and the corresponding protein sequence has been derived. Murine CTL cDNA libraries contained in the vector lambda gt11 were screened by using a monospecific antiserum to purified P1. Three recombinant phages were isolated and their cDNA inserts were sequenced. The derived protein sequence contains 554 amino acids and displays, as expected, considerable homology with certain functional domains in the complement components C9, C8 alpha, C8 beta, and C7. The identity of P1 cDNA clones was verified by prokaryotic expression and the reactivities of antisera produced to the expressed proteins. In addition, antisera were produced to two synthetic peptides located in the center and C-terminal portions of P1. All antisera reacted with purified P1. In Northern blot analyses, P1 cDNA probes recognized a 2.9-kilobase mRNA only in CTL. Perforin mRNA was found in all cloned CTL and in all mixed lymphocyte reactions that gave rise to cytotoxic cells. Perforin mRNA was also detected in virus-specific CTL that had been generated in vivo and isolated from liver tissue of mice infected with lymphocytic choriomeningitis virus. The cell-specific expression of perforin is consistent with its postulated role in cytolysis.

Characterization of three serine esterases isolated from human IL-2 activated killer cells

Human peripheral blood mononuclear cells, activated for 14 to 20 days with 1000 U/ml rIL-2, develop strong cytotoxicity for NK sensitive and resistant targets. This process is accompanied by the acquisition of cytoplasmic granules in approximately 60% of the cells and by the expression of esterase activity cleaving the synthetic substrate BLT. The esterase activity, localized in the cytoplasmic granules, was purified and characterized. Three proteins with 3H-DFP binding activity were isolated and had the following properties. Following the proposed nomenclature by Masson et al., the esterases were named human granzymes 1, 2, and 3. Human granzyme 1 on SDS-PAGE has an unreduced relative m.w. of 43,000 and can form disulfide-linked oligomers of relative higher m.w. All forms of granzyme 1 bind 3H-DFP. Upon reduction, granzyme 1 migrates with Mr 30,000 on SDS-PAGE. Additional proteolytic fragments of Mr 24,000 and Mr 28,000 are observed in some reduced preparations. Granzyme 1 cleaves the substrate BLT and appears homologous with murine granzyme A. Human granzyme 2 has an unreduced relative m.w. of 30,000; after reduction, it migrates at Mr 32,000. Even though granzyme 2 binds 3H-DFT, it does not cleave BLT. Human granzyme 2 has properties similar to those of murine granzymes B-H. Human granzyme 3 has unreduced and reduced relative m.w. of 25,000 and 28,000, respectively. It is active in cleaving the substrate BLT. A murine analog for human granzyme 3 has not been described previously. N-terminal sequencing of the purified human granzymes revealed that human granzyme 1 is the gene product of human Hanuka factor cDNA clone and that it represents the human homolog to murine granzyme A. Similarly, human granzyme 2 revealed absolute identity with cDNA-derived N-terminal sequence of a putative human lymphocyte protease cDNA clone.

Characterization of three serine esterases isolated from human IL-2 activated killer cells

Human peripheral blood mononuclear cells, activated for 14 to 20 days with 1000 U/ml rIL-2, develop strong cytotoxicity for NK sensitive and resistant targets. This process is accompanied by the acquisition of cytoplasmic granules in approximately 60% of the cells and by the expression of esterase activity cleaving the synthetic substrate BLT. The esterase activity, localized in the cytoplasmic granules, was purified and characterized. Three proteins with 3H-DFP binding activity were isolated and had the following properties. Following the proposed nomenclature by Masson et al., the esterases were named human granzymes 1, 2, and 3. Human granzyme 1 on SDS-PAGE has an unreduced relative m.w. of 43,000 and can form disulfide-linked oligomers of relative higher m.w. All forms of granzyme 1 bind 3H-DFP. Upon reduction, granzyme 1 migrates with Mr 30,000 on SDS-PAGE. Additional proteolytic fragments of Mr 24,000 and Mr 28,000 are observed in some reduced preparations. Granzyme 1 cleaves the substrate BLT and appears homologous with murine granzyme A. Human granzyme 2 has an unreduced relative m.w. of 30,000; after reduction, it migrates at Mr 32,000. Even though granzyme 2 binds 3H-DFT, it does not cleave BLT. Human granzyme 2 has properties similar to those of murine granzymes B-H. Human granzyme 3 has unreduced and reduced relative m.w. of 25,000 and 28,000, respectively. It is active in cleaving the substrate BLT. A murine analog for human granzyme 3 has not been described previously. N-terminal sequencing of the purified human granzymes revealed that human granzyme 1 is the gene product of human Hanuka factor cDNA clone and that it represents the human homolog to murine granzyme A. Similarly, human granzyme 2 revealed absolute identity with cDNA-derived N-terminal sequence of a putative human lymphocyte protease cDNA clone.

Isolation and characterization of cytotoxic granules from human lymphokine (interleukin 2) activated killer cells

Cytotoxic granules were isolated from human lymphokine-activated killer (LAK) cells and analyzed for their biochemical properties. Isolated granules of approximately 85-95% purity were obtained by differential centrifugation followed by discontinuous Percoll gradient centrifugation. The murine lymphocyte granule marker N-alpha-carbamazepine-L-lysine thiobenzyl ester-esterase as well as cytotoxic activity toward the human tumor cell lines K562, Raji, Daudi, and CEM were associated with LAK granule fractions. Granule-associated N-alpha-carbamazepine-L-lysine thiobenzyl ester-esterase activity increased in recombinant interleukin 2 expanded human LAK cells in parallel with cytotoxic activity for Raji tumor cell targets. Cytotoxic LAK cell granules mediated calcium-dependent killing of the tumor cell lines K562, Raji, Daudi, and CEM. However, no calcium-dependent hemolytic activity was found. Preincubation of human granules with calcium, a treatment which totally inactivates the hemolytic and cytotoxic activity of murine lymphocyte granules [perforin 1 (P1)] had no effect on human LAK granule cytotoxicity for nucleated cells. Human LAK granules appear to contain P1 detected as cross-reactive antigen detected by mouse anti-P1 and human anti-C9 in Western blot analysis. In addition, Northern blot analysis of polyadenylated RNA isolated from human LAK cells using a murine P1 complementary DNA probe showed a cross-hybridizing 2.8- to 3.0-kilobase mRNA species identical in size to murine P1 mRNA. These results demonstrate that despite similar biochemical composition, functional differences exist between human and murine cytotoxic granules. Human LAK granules were synthesized in response to recombinant interleukin 2 activation and appeared in parallel with cytotoxicity for tumor targets, suggesting an important role for LAK granules in tumor cell cytotoxicity by human LAK cells.

Function of granule perforin and esterases in T cell-mediated reactions. Components required for delivery of molecules to target cells

Cognate T cell-mediated functions require antigen and MHC-restricted recognition of target cells. T-effector functions comprise the delivery of signals for help, for suppression, or for cell death of the target cell. In the case of the delivery of cytotoxicity and of help for B-cell antibody production, it is known that the secretory apparatus of the effector cell participates. Prior to secretion, many components of the effector cell are stored in cytoplasmic granules. Among the important and apparently constant constituents of granules are pore-forming proteins (perforins) and proteinases (granzymes). The putative role of perforin has been thought to mediate direct cytotoxicity. It is postulated here that, in addition, perforin at low concentrations may induce target-cell endocytosis through the formation of Ca channels. Localized endocytosis of the target at the contact site in turn may lead to the uptake of locally secreted effector-cell factors, such as cytotoxic factors (CTL), lymphokines (helper cells), or suppressor factors (suppressor cells). The potential importance of such a mechanism is the delivery and uptake of secreted effector-cell components into the endosomes of target cells, bypassing the need for appropriate target-cell receptors. Perforin thus may subserve two functions depending on its intragranular concentration: one, as a killer molecule, and two, as a delivery system for additional granule factors. One of the roles of esterases in T cell-mediated cognate-effector functions may be to allow recycling of the effector cell. This apparently is achieved by an active process of detachment of the effector T cell from the target cell, possibly by way of the proteolytic cleavage of adhesion molecules. Esterases are secreted, together with perforin and other factors, during granule release at the effector target-contact site, where they can cleave intercellular adhesion molecules and thus allow effector-cell recycling and attachment to new target cells. Other roles of esterases, not discussed here, may include participation directly in the cytotoxic process through uptake into the target cell. The evidence for a common intercellular molecular delivery mechanism of cognate effector T-cell function involving perforin and esterases is summarized. This concept represents a unifying hypothesis for MHC-restricted, contact-requiring, intercellular T cell-signal delivery as well as for the delivery of cytotoxicity by non-MHC-restricted T cells and natural killer cells.