Peripheral tolerance and autoimmunity: lessons from in vivo imaging

Multi-photon microscopy has taken hold as a widely used technique in immunology, allowing for imaging of the kinetics of immune cell motility and cell-cell interactions, but what have we learned from this technique about the processes involved in peripheral tolerance and autoimmunity? Various studies have now looked at the dynamics of several mechanisms of peripheral T cell tolerance and efforts to examine the dynamics of the autoimmune response at the disease site are also under way. Here, we will discuss the findings of studies that use multi-photon microscopy to examine the dynamics of T cell tolerance in the lymph nodes and of the autoimmune processes involved in models of type 1 diabetes and multiple sclerosis. An emerging theme from these studies is that short T cell-antigen presenting cell interactions can lead to tolerance, and that autoreactive T cell restimulation at the disease site can play an important role in autoimmune disease exacerbation.

Evolving immune circuits are generated by flexible, motile, and sequential immunological synapses

The immune system is made up of a diverse collection of cells, each of which has distinct sets of triggers that elicit unique and overlapping responses. It is correctly described as a 'system' because its overall properties (e.g. 'tolerance', 'allergy') emerge from multiple interactions of its components cells. To mobilize a response where needed, the majority of the cells of the system are obligatorily highly motile and so must communicate with one another over both time and space. Here, we discuss the flexibility of the primary immunological synapse (IS) with respect to motility. We then consider the primary IS as an initiating module that licenses 'immunological circuits': the latter consisting of two or more cell-cell synaptic interactions. We discuss how two or three component immunological circuits interact might with one another in sequence and how the timing, stoichiometry, milieu, and duration of assembly of immunological circuits are likely to be key determinants in the emergent outcome and thus the system-wide immune response. An evolving consideration of immunological circuits, with an emphasis on the cell-cell modules that complement T-antigen-presenting cell interaction, provides a fundamental starting point for systems analysis of the immune response.

Real-time analysis of T cell receptors in naive cells in vitro and in vivo reveals flexibility in synapse and signaling dynamics

Real-time imaging defines the dynamics of TCR and T cell motility during early T cell activation in lymph nodes.

The real-time dynamics of the T cell receptor (TCR) reflect antigen detection and T cell signaling, providing valuable insight into the evolving events of the immune response. Despite considerable advances in studying TCR dynamics in simplified systems in vitro, live imaging of subcellular signaling complexes expressed at physiological densities in intact tissues has been challenging. In this study, we generated a transgenic mouse with a TCR fused to green fluorescent protein to provide insight into the early signaling events of the immune response. To enable imaging of TCR dynamics in naive T cells in the lymph node, we enhanced signal detection of the fluorescent TCR fusion protein and used volumetric masking with a second fluorophore to mark the T cells expressing the fluorescent TCR. These in vivo analyses and parallel experiments in vitro show minimal and transient incorporation of TCRs into a stable central supramolecular activating cluster (cSMAC) structure but strong evidence for rapid, antigen-dependent TCR internalization that was not contingent on T cell motility arrest or cSMAC formation. Short-lived antigen-independent TCR clustering was also occasionally observed. These in vivo observations demonstrate that varied TCR trafficking and cell arrest dynamics occur during early T cell activation.

Amplification of autoimmune response through induction of dendritic cell maturation in inflamed tissues

Dendritic cells (DCs) are essential in T cell-mediated destruction of insulin-producing beta cells in the islets of Langerhans in type 1 diabetes. In this study, we investigated T cell induction of intra-islet DC maturation during the progression of the disease in both autoimmune-prone NOD and resistant C57BL/6 mice. We demonstrated steady-state capture and retention of unprocessed beta cell-derived proteins by semimature intra-islet DCs in both mouse strains. T cell-mediated intra-islet inflammation induced an increase in CD40 and CD80 expression and processing of captured Ag by resident DCs without inducing the expression of the p40 subunit of IL-12/23. Some of the CD40(high) intra-islet DCs up-regulated CCR7, and a small number of CD40(high) DCs bearing unprocessed islet Ags were detected in the pancreatic lymph nodes in mice with acute intra-islet inflammation, demonstrating that T cell-mediated tissue inflammation augments migration of mature resident DCs to draining lymph nodes. Our results identify an amplification loop during the progression of autoimmune diabetes, in which initial T cell infiltration leads to rapid maturation of intra-islet DCs, their migration to lymph nodes, and expanded priming of more autoreactive T cells. Therapeutic interventions that intercept this process may be effective at halting the progression of type 1 diabetes.

Real-time imaging of TCR dynamics in vivo reveals rapid TCR clustering and internalization in response to antigen (33.13)

Real-time in vivo imaging has given insight into the cellular dynamics of T cell activation, but less is known about receptor and signaling dynamics in vivo. Live imaging of subcellular signaling complexes has been challenging in living tissues due to low physiological molecular densities, inherent limitations of fluorophores, and strong autofluorescence of tissues. Our goal was to characterize T cell receptor (TCR) dynamics in naïve T cells in the lymph node during their first encounter with dendritic cells (DCs) bearing cognate antigen. To do so we developed a mouse expressing a GFP tagged OT-I TCR to track surface and intracellular TCR, and used image processing facilitated by co-labeling of the T cells to isolate specific fluorescence. In the absence of antigen, the TCR was mainly confined to the cell surface without consistent polarity; however, a small percentage of T cells transiently clustered their TCR. Within 30 min of antigen recognition, many T cells stopped crawling and clustered their TCR. Surprisingly, detectable antigen-dependent TCR clustering duration was short (7.1±5.1 min) and as TCR clustering declined, TCR internalization increased. Thus, naïve T cells in the lymph node can signal through the TCR in response to their first interaction with DCs bearing antigen and these contacts lead to rapid TCR clustering and internalization.

This work was funded by the Larry L. Hillblom Foundation and NIH R21.

T cell trans-endothelial migration and homing to lymph nodes rely on Myosin-IIA mediated acto-myosin contractility (94.25)

T lymphocytes migrate from the vasculature into lymph nodes and tissues scanning for antigenic peptides on cell surfaces. How T cells migrate through different environments and switch between types of motility in response to different stimuli is not fully understood. We have previously shown that in vitro Myosin-IIA has a role in T cell migration by regulating the switch between fast amoeboid-like motility and slower mesenchymal-like motility. Here, using conditional knockout mice, we have studied the role of Myosin-IIA-dependent cytoskeletal contractility in the migration of primary T cells in vivo. While Myosin-IIA depleted T cells in the lymph node were motile, they did not achieve the same fast motility rates of control T cells. In addition, homing of T cells to lymph nodes was inhibited in the absence of Myosin-IIA. This resulted from inefficient trans-endothelial migration (TEM) of Myosin-IIA depleted T cells; specifically, Myosin-IIA contractility was important to squeeze the body of the T cell (containing the nucleus) through the endothelial cell layer. Overall, our results suggest a critical role for Myosin-IIA in promoting TEM and migration through constrictive environments, while migration within more 'permissive' environments is not strictly dependent on the function of this myosin motor.

This work was supported by the NIH (RO1AI052116) and the Leukemia and Lymphoma Society.

Live imaging of peripheral T cell-DC interactions in the islets: An emerging axis of autoimmune infiltration (48.15)

Defining processes of islet infiltration and destruction in Type 1 diabetes (T1D) is important for its prevention and preventing destruction of transplanted islets in diabetic individuals. We hypothesize that T1D onset and persistence is maintained by T cell-dendritic cell (DC) interactions in both the draining pancreatic lymph node (PLN) and the islets. In the steady-state, we found that intra-islet DCs are a uniform semi-mature population that constitutively take up β cell antigens in preparation for antigen presentation. Islet-antigen specific CD8 T cells activated in the PLN trafficked to the islets where they induced maturation of islet DCs. Leukocytic infiltration of the islets and islet DC maturation were dependent on IFN-γ production by T cells. Blockade of antigen processing following T cell activation in the PLN also blocked islet DC maturation but not islet infiltration. This suggests that the DCs require productive antigen-presenting interactions with T cells in the islets for maturation to occur. Through 2-photon imaging of the islets, we visualized sustained interactions of islet-antigen specific CD8 T cells with islet DCs and characterized features of cytotoxicity and dwell-time in this setting. Taken together, these data suggest that directed and synaptic delivery of IFN-γ to intra-islet DCs induces their maturation.

This work was funded by the Larry L. Hillblom Foundation and the Juvenile Diabetes Research Foundation.

Two-photon imaging of the immune system: a custom technology platform for high-speed, multicolor tissue imaging of immune responses

Modern imaging approaches are proving important for addressing contemporary issues in the immune system. These approaches are especially useful for characterizing the complex orchestration of immune responses in vivo. Multicolor, two-photon imaging has been proven to be especially enabling for such studies because of its superior tissue penetration, reduced image degradation by light scattering leading to better resolution, and its high image quality deep inside tissues. Here, we examine the functional requirements of two-photon imaging instruments necessary for such immune studies. These requirements include frame rate, spatial resolution and the number of emission channels. We use this discussion as a starting point to compare commercial systems and to introduce a custom technology platform that meets those requirements. This platform is noteworthy because it is very cost-effective, flexible and experimentally useful. Representative data collected with this instrument is used to demonstrate the utility of this platform. Finally, as the field is rapidly evolving, consideration is given to some of the cutting-edge developments in multiphoton microscopy that will likely improve signal strength, depth penetration and/or the experimental usefulness of this approach.

A synaptic basis for paracrine interleukin-2 signaling during homotypic T cell interaction

T cells slow their motility, increase adherence, and arrest after encounters with antigen-presenting cells (APCs) bearing peptide-MHC complexes. Here, we analyzed the cell-cell communication among activating T cells. In vivo and in vitro, activating T cells associated in large clusters that collectively persisted for >30 min, but they also engaged in more transient interactions, apparently distal to APCs. Homotypic aggregation was driven by LFA-1 integrin interactions. Ultrastructural analysis revealed that cell-cell contacts between activating T cells were organized as multifocal synapses, and T cells oriented both the microtubule-organizing complex and interleukin-2 (IL-2) secretion toward this synapse. T cells engaged in homotypic interactions more effectively captured IL-2 relative to free cells. T cells receiving paracrine synaptic IL-2 polarized their IL-2 signaling subunits into the synaptic region and more efficiently phosphorylated the transcription factor STAT5, likely through a synapse-associated signaling complex. Thus, synapse-mediated cytokine delivery accelerates responses in activating T cells.

Deletional tolerance mediated by extrathymic Aire-expressing cells

The prevention of autoimmunity requires the elimination of self-reactive T cells during their development and maturation. The expression of diverse self-antigens by stromal cells in the thymus is essential to this process and depends, in part, on the activity of the autoimmune regulator (Aire) gene. Here we report the identification of extrathymic Aire-expressing cells (eTACs) resident within the secondary lymphoid organs. These stromally derived eTACs express a diverse array of distinct self-antigens and are capable of interacting with and deleting naïve autoreactive T cells. Using two-photon microscopy, we observed stable antigen-specific interactions between eTACs and autoreactive T cells. We propose that such a secondary network of self-antigen-expressing stromal cells may help reinforce immune tolerance by preventing the maturation of autoreactive T cells that escape thymic negative selection.

Surface-bound chemokines capture and prime T cells for synapse formation

T cell activation in vivo occurs in a lymphoid milieu that presents chemotactic and T cell receptor signals concurrently. Here we demonstrate that T cell zone chemokines such as CCL21 are bound to the surface of lymph node dendritic cells. Contact with antigen-presenting cells bearing chemokines costimulated T cells by a previously unknown two-step contact mechanism. T cells initially formed an antigen-independent 'tethered' adhesion on chemokine-bearing antigen-presenting cells. The formation of those tethers superseded T cell receptor signaling and immunological synapse formation. However, chemokine-tethered T cells were hyper-responsive to subsequent contacts with antigen-presenting cells. Thus, T cells are costimulated 'in trans' and sequentially after initial engagement with their chemokine-rich environment.

Mechanisms of T cell motility and arrest: deciphering the relationship between intra- and extracellular determinants

T lymphocytes are capable of rapid motility in vitro and in vivo. Upon antigen recognition, they may stop crawling and form a stable cell-cell contact called the 'immunological synapse' (IS). However, it is becoming clear that this outcome may not occur with the reliability that was once presumed. T cells, particularly naïve cells, are apparently triggered partly 'on the fly' during short contacts with peptide-MHC (pMHC) bearing antigen-presenting cells (APCs) and are also influenced in both activity and synapse duration by a multitude of external cues. Underlying the emerging issues is a paucity of data concerning the cell biology of T lymphocytes. Here, we review the molecular mechanisms of crawling and adhesion versus the various potential modes of 'stopping' in T lymphocytes. Both motility and arrest involve similar processes: adhesion, actin elongation and internal tension control, but with different coordination. We will attempt to integrate this with the known and potential external cues that signal for T cell motility versus stopping to form a synapse in vivo. Finally, we discuss how this interplay may give rise to unexpectedly complex motile and morphological behavior.

Identification of naturally processed CD8 T cell epitopes from prostein, a prostate tissue-specific vaccine candidate

Prostein is a prostate tissue-specific protein that is uniquely and abundantly expressed in normal and cancerous prostate tissues. Due to this expression profile, we examined the immunogenicity of prostein as a potential vaccine candidate for prostate cancer. To determine the presence of CD8 T cells specific for naturally processed prostein-derived epitopes in healthy individuals, we developed and applied an in vitro stimulation protocol. Using this protocol, we identified CD8 T cells specific for prostein in the peripheral blood of a male and a female donor. Prostein-specific CD8 T cell clones specifically recognized prostein-expressing targets, including prostate tumor cell lines expressing the relevant HLA alleles. CD8 T cell clones isolated from the male donor were significantly less effective in recognizing target cells compared to cells isolated from the female donor and appeared to recognize subdominant epitopes. The identification of a prostein-specific CD8 T cell repertoire supports the development of prostein in vaccination strategies against prostate cancer. Furthermore, the naturally processed peptide epitopes identified provide tools for the development of peptide-based vaccination strategies against prostate cancer and for monitoring of prostein-specific responses in vaccinated patients.

Molecular and Immunological Evaluation of the Transcription Factor SOX-4 as a Lung Tumor Vaccine Antigen

The developmental transcription factor SOX-4 has been shown to be highly and differentially overexpressed in primary small cell lung carcinomas (SCLC). To examine the potential of SOX-4 for broad use as a lung cancer vaccine, we have evaluated the expression of SOX-4 in a panel of primary adenocarcinoma, squamous, and large cell tumor samples as well as in a panel of established small cell and non-small cell lung carcinoma tumor cell lines. SOX-4 mRNA is shown to be overexpressed in a substantial fraction of each of these lung tumor types. To examine the immunological potential of SOX-4, we have evaluated the presence of SOX-4-specific CD4 and CD8 T cells in PBMC of healthy donors and the presence of SOX4-specific Abs in sera from SCLC patients. We demonstrate the presence of both CD4 and CD8 T cells that recognize naturally processed epitopes derived from SOX-4 as well as the presence of SOX-4-specific Abs in sera from SCLC patients, but not in sera from healthy donors. The lung tumor-specific overexpression and demonstration of a comprehensive Ag-specific immune response specific for SOX-4 support the use of this molecule in the development of whole gene-, peptide-, or protein-based vaccination strategies against lung cancer. Furthermore, the identification of naturally processed T cell and Ab epitopes from SOX-4 provides valuable tools for the development of peptide-based vaccination strategies against lung cancer as well as to monitor SOX-4-specific responses in vaccinated patients.

Identification of naturally processed CD4 T cell epitopes from the prostate-specific antigen kallikrein 4 using peptide-based in vitro stimulation

Kallikrein (KLK)4 is a recently described member of the tissue kallikrein gene family that is specifically expressed in normal and prostate tumor tissues. The tissue-specific expression profile of this molecule suggests that it might be useful as a vaccine candidate against prostate cancer. To examine the presence of CD4 T cells specific for KLK4 in PBMC of normal individuals, a peptide-based in vitro stimulation protocol was developed that uses overlapping KLK4-derived peptides spanning the majority of the KLK4 protein. Using this methodology, three naturally processed CD4 epitopes derived from the KLK4 sequence are identified. These epitopes are restricted by HLA-DRB1*0404, HLA-DRB1*0701, and HLA-DPB1*0401 class II alleles. CD4 T cell clones specific for these epitopes are shown to efficiently and specifically recognize both recombinant KLK4 protein and lysates from prostate tumor cell lines virally infected to express KLK4. CD4 T cells specific for these KLK4 epitopes are shown to exist in PBMC from multiple male donors that express the relevant class II alleles, indicating that a CD4 T cell repertoire specific for KLK4 is present and potentially expandable in prostate cancer patients. The demonstration that KLK4-specific CD4 T cells exist in the peripheral circulation of normal male donors and the identification of naturally processed KLK4-derived CD4 T cell epitopes support the use of KLK4 in whole gene-, protein-, or peptide-based vaccine strategies against prostate cancer. Furthermore, the identification of naturally processed KLK4-derived epitopes provides valuable tools for monitoring preexisting and vaccine-induced responses to this molecule.

The effects of promotion and prevention cues on creativity

This study tested whether cues associated with promotion and prevention regulatory foci influence creativity. The authors predicted that the "risky," explorative processing style elicited by promotion cues, relative to the risk-averse, perseverant processing style elicited by prevention cues, would facilitate creative thought. These predictions were supported by two experiments in which promotion cues bolstered both creative insight (Experiment 1) and creative generation (Experiment 2) relative to prevention cues. Experiments 3 and 4 provided evidence for the process account of these findings. suggesting that promotion cues, relative to prevention cues, produce a riskier response bias (Experiment 3) and bolster memory search for novel responses (Experiment 4). A final experiment provided evidence that individual differences in regulatory focus influence creative problem solving in a manner analogous to that of incidental promotion and prevention cues.

The effects of promotion and prevention cues on creativity

This study tested whether cues associated with promotion and prevention regulatory foci influence creativity. The authors predicted that the "risky," explorative processing style elicited by promotion cues, relative to the risk-averse, perseverant processing style elicited by prevention cues, would facilitate creative thought. These predictions were supported by two experiments in which promotion cues bolstered both creative insight (Experiment 1) and creative generation (Experiment 2) relative to prevention cues. Experiments 3 and 4 provided evidence for the process account of these findings. suggesting that promotion cues, relative to prevention cues, produce a riskier response bias (Experiment 3) and bolster memory search for novel responses (Experiment 4). A final experiment provided evidence that individual differences in regulatory focus influence creative problem solving in a manner analogous to that of incidental promotion and prevention cues.

Induction of human immunodeficiency virus (HIV)-specific CD8 T-cell responses by Listeria monocytogenes and a hyperattenuated Listeria strain engineered to express HIV antigens

Induction of cell-mediated immunity may be essential for an effective AIDS vaccine. Listeria monocytogenes is an attractive bacterial vector to elicit T-cell immunity to human immunodeficiency virus (HIV) because it specifically infects monocytes, key antigen-presenting cells, and because natural infection originates at the mucosa. Immunization with recombinant L. monocytogenes has been shown to protect mice from lymphocytic choriomeningitis virus, influenza virus, and tumor inoculation. L. monocytogenes expressing HIV gag elicits sustained high levels of Gag-specific cytotoxic T lymphocytes (CTLs) in mice. We have examined the ability of Listeria to infect human monocytes and present HIV antigens to CD8 T lymphocytes of HIV-infected donors to induce a secondary T-cell immune response. Using this in vitro vaccination protocol, we show that L. monocytogenes expressing the HIV-1 gag gene efficiently provides a strong stimulus for Gag-specific CTLs in HIV-infected donor peripheral blood mononuclear cells. Listeria expressing Nef also elicits a secondary in vitro anti-Nef CTL response. Since L. monocytogenes is a pathogen, before it can be seriously considered as a human vaccine vector, safety concerns must be addressed. We therefore have produced a highly attenuated strain of L. monocytogenes that requires D-alanine for viability. The recombinant bacteria are attenuated at least 10(5)-fold. We show that when these hyperattenuated bacteria are engineered to express HIV-1 Gag, they are at least as efficient at stimulating Gag-specific human CTLs in vitro as wild-type recombinants. These results suggest that attenuated Listeria is an attractive candidate vaccine vector to induce T-cell immunity to HIV in humans.

The effects of approach and avoidance motor actions on the elements of creative insight

The authors propose that the nonaffective bodily feedback produced by arm flexion and extension informs individuals about the processing requirements of the situation, leading to the adoption of differential processing styles and thereby influencing creativity. Specifically, the authors predicted that arm flexion would elicit a heuristic processing strategy and bolster insight processes, whereas arm extension would elicit a systematic processing strategy and impair insight processes. To test these predictions, the authors assessed the effects of these motor actions on 3 central elements of creative insight: contextual set-breaking, restructuring, and mental search. As predicted, in 6 experiments, arm flexion, relative to arm extension, facilitated insight-related processes. In a 7th experiment, arm extension, relative to arm flexion, facilitated analytical reasoning, supporting a cognitive tuning interpretation of the findings.

CD3zeta and CD28 down-modulation on CD8 T cells during viral infection

Down-modulation of CD3zeta expression on CD8 T lymphocytes occurs, independently of other T-cell receptor (TCR)-CD3 components, in tumor-infiltrating lymphocytes, human immunodeficiency virus infection, and autoimmune disease. These associations suggest that it might be related to chronic antigenic stimulation. CD3zeta down-modulation was found, however, in CD8 T cells that proliferate in response to acute viral infections. In 3 otherwise healthy donors with acute gastroenteritis, infectious mononucleosis, and Epstein-Barr virus/cytomegalovirus/mononucleosis, 30% to 60% of circulating CD8 T cells had down-modulated CD3zeta to below the level of detection. The CD3zeta-T cells were also CD28- but expressed the activation markers HLA-DR and CD57. CD3zeta-CD28- T cells are effector CTL because they express perforin and produce IFN-gamma, but not IL-2, on activation and contain the viral-specific cytotoxic T lymphocyte (CTL). However, CD3zeta-CD28-T cells generally do not express CD25 after anti-CD3 and anti-CD28 stimulation and are not cytotoxic until they are cultured with IL-2 overnight. Cytotoxicity coincides with the re-expression of CD3zeta but not CD28. Down-modulation of CD3zeta and CD28 on effector CTL may control CTL triggering and proliferation to prevent immunopathogenesis.

Expansion of CD57 and CD62L-CD45RA+ CD8 T lymphocytes correlates with reduced viral plasma RNA after primary HIV infection

OBJECTIVE

CD8 T cells, expressing cell surface molecules distinct from those on resting and naive T cells, are increased in HIV infection. The association of increased CD38 and human leukocyte antigen DR (HLA-DR) CD8 T cells with poor prognosis has suggested that activated CD8 T cells may aggravate HIV infection. We examined whether other immunological parameters might influence the viral setpoint.

DESIGN

Peripheral T cells from nine untreated patients, obtained after primary HIV infection when plasma HIV had stabilized, were examined for proteins expressed in activated versus resting, memory versus naive, and cytolytic versus non-cytolytic T cells.

METHODS

The proportion of CD8 T cells that stain for CD38 and HLA-DR, CD28 and CD57 was compared with plasma viraemia and CD4 cell count. These parameters were also compared with the proportion of CD4 and CD8 T cells that express CD62L and CD45RA, present on naive cells and down-modulated in memory cells. Internal staining for the cytotoxic protein granzyme A was also examined.

RESULTS

An increase in CD38 and CD38 HLA-DR CD8 T cells correlated with increased plasma viral RNA (P < 0.00002, P < 0.03, respectively). An increase in CD8 T cells expressing granzyme A was associated with lower CD4 cell counts (P < 0.04). However, the expansion of CD57 and CD62L CD45RA+ CD8 T cells was associated with a lower viral setpoint (P < 0.01, P < 0.02, respectively).

CONCLUSION

Phenotypically defined activated CD8 T cells may have different functions in HIV infection. Activated CD8 T cells that are CD57 or CD62L(-)CD45RA+ may be beneficial, because their expansion in untreated patients correlates with a reduced viral setpoint after primary infection.

A role for heat shock protein 27 in CTL-mediated cell death

CTL exocytosis of granules containing perforin and granzyme proteases induces apoptotic cell death. Either granzyme A or B can act with perforin to trigger apoptosis. Granzyme B activates a ubiquitous apoptotic cascade induced by caspase cleavage, but the granzyme A pathway is largely unknown. Using affinity chromatography with recombinant mutant inactive granzyme A, we previously isolated two granzyme A-binding proteins, PHAP (putative HLA-associated protein) I and II. PHAP II, a substrate of granzyme A, is degraded within minutes of CTL attack. Two additional cytoplasmic proteins of 27 and 53 kDa bind strongly to the mutant granzyme A column, requiring 6 M urea to elute. Sequencing identified these as the monomer and dimer of hsp27, a small heat shock protein up-regulated by stress and cellular activation. Hsp27 coprecipitates with granzyme A from cytoplasmic lysates and is not a substrate of the enzyme. Hsp27 translocates to the detergent-insoluble fraction of target cells and relocalizes from diffuse cytoplasmic staining to long filamentous fibers, especially concentrated in a perinuclear region, within minutes of CTL attack. Hsp27 may participate in morphologic changes during granule-mediated lysis. Low or absent levels of hsp27 expression in T lymphocytes, even after heat shock, may play a role in CTL resistance to granule-mediated lysis.

A Role for Heat Shock Protein 27 in CTL-Mediated Cell Death

CTL exocytosis of granules containing perforin and granzyme proteases induces apoptotic cell death. Either granzyme A or B can act with perforin to trigger apoptosis. Granzyme B activates a ubiquitous apoptotic cascade induced by caspase cleavage, but the granzyme A pathway is largely unknown. Using affinity chromatography with recombinant mutant inactive granzyme A, we previously isolated two granzyme A-binding proteins, PHAP (putative HLA-associated protein) I and II. PHAP II, a substrate of granzyme A, is degraded within minutes of CTL attack. Two additional cytoplasmic proteins of 27 and 53 kDa bind strongly to the mutant granzyme A column, requiring 6 M urea to elute. Sequencing identified these as the monomer and dimer of hsp27, a small heat shock protein up-regulated by stress and cellular activation. Hsp27 coprecipitates with granzyme A from cytoplasmic lysates and is not a substrate of the enzyme. Hsp27 translocates to the detergent-insoluble fraction of target cells and relocalizes from diffuse cytoplasmic staining to long filamentous fibers, especially concentrated in a perinuclear region, within minutes of CTL attack. Hsp27 may participate in morphologic changes during granule-mediated lysis. Low or absent levels of hsp27 expression in T lymphocytes, even after heat shock, may play a role in CTL resistance to granule-mediated lysis.