Staphylococcal superantigens induce lymphotactin production by human CD4+ and CD8+ T cells

Specific binding-induced modulation of the XCL1 metamorphic equilibrium

The metamorphic protein XCL1 switches between two distinct native structures with different functions in the human immune system. This structural interconversion requires complete rearrangement of all hydrogen bonding networks, yet fold-switching occurs spontaneously and reversibly in solution. One structure occupies the canonical α-β chemokine fold and binds XCL1's cognate G-protein coupled receptor, while the other structure occupies a dimeric, all-β fold that binds glycosaminoglycans and has antimicrobial activity. Both of these functions are important for the biologic role of XCL1 in the immune system, and each structure is approximately equally populated under near-physiologic conditions. Recent work has begun to illuminate XCL1's role in combatting infection and cancer. However, without a way to control XCL1's dynamic structural interconversion, it is difficult to study the role of XCL1 fold-switching in human health and disease. Thus, a molecular tool that can regulate the fractional population of the two XCL1 structures is needed. Here, we find by heparin affinity chromatography and NMR that an engineered XCL1 variant called CC5 can trigger a dose-dependent shift in XCL1's metamorphic equilibrium such that the receptor binding structure is depleted, and the antimicrobial structure is more heavily populated. This shift likely occurs due to formation of XCL1-CC5 heterodimers in which both protomers occupy the β-sheet structure. These findings lay the groundwork for future studies seeking to understand the functional role of XCL1 metamorphosis, as well as studies screening for a drug-like molecule that can therapeutically target XCL1 by tuning its metamorphic equilibrium. Moreover, the proof of concept presented here suggests that protein metamorphosis is druggable, opening numerous avenues for controlling biological function of metamorphic proteins by altering the population of their multiple native states.

Diepoxybutane induces the expression of a novel p53-target gene XCL1 that mediates apoptosis in exposed human lymphoblasts

Diepoxybutane (DEB) is the most potent active metabolite of the environmental chemical 1,3-butadiene (BD). BD is a human carcinogen that exhibits multiorgan systems toxicity. Our previous studies demonstrated that the X-C motif chemokine ligand 1 (XCL1) gene expression was upregulated 3.3-fold in a p53-dependent manner in TK6 lymphoblasts undergoing DEB-induced apoptosis. The tumor-suppressor p53 protein is a transcription factor that regulates a wide variety of cellular processes, including apoptosis, through its various target genes. Thus, the objective of this study was to determine whether XCL1 is a novel direct p53 transcriptional target gene and deduce its role in DEB-induced toxicity in human lymphoblasts. We utilized the bioinformatics tool p53scan to search for known p53 consensus sequences within the XCL1 promoter region. The XCL1 gene promoter region was found to contain the p53 consensus sequences 5'-AGACATGCCTAGACATGCCT-3' at three positions relative to the transcription start site (TSS). Furthermore, the XCL1 promoter region was found, through reporter gene assays, to be transactivated at least threefold by wild-type p53 promoter in DEB-exposed human lymphoblasts. Inactivation of the XCL1 promoter p53-binding motif located at -2.579 kb relative to TSS reduced the transactivation function of p53 on this promoter in DEB-exposed cells by 97%. Finally, knockdown of XCL1 messenger RNA with specific small interfering RNA inhibited DEB-induced apoptosis in human lymphoblasts by 50%. These observations demonstrate, for the first time, that XCL1 is a novel DEB-induced direct p53 transcriptional target gene that mediates apoptosis in DEB-exposed human lymphoblasts.

A Requirement for Metamorphic Interconversion in the Antimicrobial Activity of Chemokine XCL1

Chemokines make up a superfamily of ∼50 small secreted proteins (8-12 kDa) involved in a host of physiological processes and disease states, with several previously shown to have direct antimicrobial activity comparable to that of defensins in efficacy. XCL1 is a unique metamorphic protein that interconverts between the canonical chemokine fold and a novel all-β-sheet dimer. Phylogenetic analysis suggests that, within the chemokine family, XCL1 is most closely related to CCL20, which exhibits antibacterial activity. The in vitro antimicrobial activity of WT-XCL1 and structural variants was quantified using a radial diffusion assay (RDA) and in solution bactericidal assays against Gram-positive and Gram-negative species of bacteria. Comparisons of WT-XCL1 with variants that limit metamorphic interconversion showed a loss of antimicrobial activity when restricted to the conserved chemokine fold. These results suggest that metamorphic folding of XCL1 is required for potent antimicrobial activity.

The CD8-derived chemokine XCL1/lymphotactin is a conformation-dependent, broad-spectrum inhibitor of HIV-1

CD8+ T cells play a key role in the in vivo control of HIV-1 replication via their cytolytic activity as well as their ability to secrete non-lytic soluble suppressive factors. Although the chemokines that naturally bind CCR5 (CCL3/MIP-1α, CCL4/MIP- 1β, CCL5/RANTES) are major components of the CD8-derived anti-HIV activity, evidence indicates the existence of additional, still undefined, CD8-derived HIV-suppressive factors. Here, we report the characterization of a novel anti-HIV chemokine, XCL1/lymphotactin, a member of the C-chemokine family that is produced primarily by activated CD8+ T cells and behaves as a metamorphic protein, interconverting between two structurally distinct conformations (classic and alternative). We found that XCL1 inhibits a broad spectrum of HIV-1 isolates, irrespective of their coreceptor-usage phenotype. Experiments with stabilized variants of XCL1 demonstrated that HIV-1 inhibition requires access to the alternative, all-β conformation, which interacts with proteoglycans but does not bind/activate the specific XCR1 receptor, while the classic XCL1 conformation is inactive. HIV-1 inhibition by XCL1 was shown to occur at an early stage of infection, via blockade of viral attachment and entry into host cells. Analogous to the recently described anti-HIV effect of the CXC chemokine CXCL4/PF4, XCL1-mediated inhibition is associated with direct interaction of the chemokine with the HIV-1 envelope. These results may open new perspectives for understanding the mechanisms of HIV-1 control and reveal new molecular targets for the design of effective therapeutic and preventive strategies against HIV-1.

Although HIV, the causative agent of AIDS, establishes a lifelong infection that cannot be eradicated even with effective treatment, the host immune system has the ability to contain its replication for many years in which the disease remains asymptomatic. Key players in HIV control are CD8+ T cells, specialized immune cells that can not only destroy infected cells, but also secrete soluble factors that suppress the virus without killing infected cells. CD8+ T cells produce multiple HIV-suppressive factors, including certain chemokines (soluble proteins that attract immune cells), which block the virus even before it can gain access to its target cells. In the present study, we characterize a new anti-HIV chemokine, XCL1 or lymphotactin, which is primarily produced by CD8+ T cells. A unique feature of XCL1 is that, unlike other antiviral chemokines, it has a very broad spectrum of activity against different variants of HIV-1 and directly binds the virus outer coat, rather than blocking specific receptors on the target cell. Also unique is that fact that XCL1 adopts two possible conformations, and only one of them is capable of HIV inhibition. These findings may open new avenues for the design of effective drugs or vaccines against HIV.

Expressions of lymphotactin and its receptor, XCR, in Lewis rats with experimental autoimmune anterior uveitis

BACKGROUND

To demonstrate the expression of lymphotactin and its receptor (XCR) in the iris/ciliary body and popliteal lymph node, and to clarify their roles in experimental autoimmune anterior uveitis (EAAU).

METHODS

Uveitis was induced in Lewis rats by injection of melanin-associated antigen into the peritoneum and footpad. At defined time points, mRNA expression levels of lymphotactin and XCR in the iris/ciliary body and popliteal lymph node were measured by semiquantitative polymerase chain reaction. Lymphotactin levels in aqueous humor and serum after immunization were determined by enzyme-linked immunosorbent assay. In a separate experiment, an NF-κB inhibitor, pyrrolidine dithiocarbamate (PDTC; 200 mg/kg/day), was injected daily into the intraperitoneum after immunization. Cellular sources of lymphotactin were determined by immunhistochemical staining and flow cytometry.

RESULTS

Lymphotactin mRNA was upregulated in the iris/ciliary body with a peak level at day 14, which is in line with the disease course. XCR mRNA was expressed maximally and then declined gradually from days 5 to 21. With an expression pattern similar to that of mRNA expression, lymphotactin in aqueous humor had attracted corresponding numbers of leukocytes. PDTC markedly inhibited the expression of lymphotactin in aqueous humor and serum. Immunohistochemical staining and flow cytometry analysis revealed that the expression of lymphotactin was detected in infiltrated inflammatory cells, dominantly CD8+ T cells, and increased along with inflammation.

CONCLUSIONS

The lymphotactin and XCR interaction might direct distinct lymphocytes subsets to inflammatory sites. Lymphotactin could regulate the inflammatory process. Lymphotactin expression may be modulated, at least in part, through the NF-κB signaling pathway.

Chapter 3. Lymphotactin structural dynamics

Lymphotactin/XCL1, the defining member of the C class of chemokines, undergoes a conformational change that involves the complete restructuring of all stabilizing interactions. Other chemokines are restricted to a single conformation by a pair of conserved disulfide crosslinks, one of which is absent in lymphotactin. This structural interconversion is entirely reversible, and the two-state equilibrium is sensitive to changes in temperature and ionic strength. One species adopts the conserved chemokine fold as a monomer and functions as an agonist for XCR1, the specific G-protein-coupled receptor for lymphotactin. Rearrangement to the other conformation produces a novel four-stranded sheet that dimerizes to form a beta sandwich with high affinity for cell-surface glycosaminoglycans. We developed methods for resolving the two species and investigated the dynamics of human lymphotactin structural interconversion with NMR spectroscopy, heparin affinity chromatography, and time-resolved fluorescence on the wild-type protein and a panel of amino acid-substituted lymphotactin variants. Our results show that the lymphotactin structural rearrangement occurs at a rate of approximately 1/s and that mutation of residues required for glycosaminoglycan binding shifts the conformational equilibrium toward the chemokine-like fold. We speculate that charge repulsion between arginines 23 and 43 destabilizes the chemokine fold and promotes conversion to the novel lymphotactin dimer, whereas binding of chloride or another anion stabilizes the chemokine fold by neutralizing the repulsive effect.

Staphylococcal superantigens of the enterotoxin gene cluster (egc) for treatment of stage IIIb non-small cell lung cancer with pleural effusion

There has been renewed interest in the superantigens as antitumor agents with the discovery of a group of bacterial superantigens known as the enterotoxin gene cluster (egc staphylococcal enterotoxins [SEs]). This article discusses the mechanisms by which egc SEs induce tumor killing and pleurodesis. The application of SE homolog and nucleic acid compositions as vaccines and for treatment of established tumors is reviewed. Finally, the use of native SEs ex vivo-intratumorally and intravesicularly administered superantigens against established tumors-is described and the interrelation between superantigen therapy and chemoradiotherapy.

Intrapleural Staphylococcal Superantigen Induces Resolution of Malignant Pleural Effusions and a Survival Benefit in Non-Small Cell Lung Cancer

BACKGROUND

Malignant pleural effusion (MPE) may occur in up to 50% of patients with non-small cell lung cancer (NSCLC). The majority of these patients have a poor performance status and a dismal prognosis, with survival duration ranging from 2 to 3 months. Since these patients are typically symptomatic from their MPE, prompt treatment is required. Patients with symptomatic MPE from NSCLC and poor performance scores (Eastern Cooperative Oncology Group [ECOG] score >/= 2, Karnofsky performance status [KPS] score < 50) are generally not offered systemic chemotherapy. Treatment is palliative and includes intrapleural catheter drainage or chemical pleurodesis with talc, doxycycline, or bleomycin. None of the latter modalities prolong survival.

OBJECTIVE

Our goal was to investigate the toxicity and therapeutic effect of a new therapeutic agent, Staphylococcus aureus superantigen (SSAg), a powerful T-cell stimulant administered intrapleurally to unselected, consecutive patients with MPE from NSCLC (stage IIIb with pleural effusion) and a poor performance status. By providing direct access of the SSAg to the bronchial and mediastinal lymphatics, we predicted that intrapleural administration of SSAg would induce resolution of MPE and prolong survival in this population with advanced NSCLC and a limited prognosis.

METHODS

Fourteen consecutive, unselected patients with MPE from NSCLC and a median pretreatment KPS score of 40 (range, 10 to 60) received pleural instillation of SSAg, 100 to 400 pg, once or twice weekly (mean, 3.7 +/- 1.3 treatments [+/- SD]) until the pleural effusions resolved. They were evaluated for drug toxicity, resolution, duration of MPE, and survival.

RESULTS

Other than mild fever (maximum grade 2), toxicity of SSAg treatment was trivial and notably devoid of respiratory distress or hypotension. Eleven patients had a complete response (CR), and 3 patients had a partial response of their MPE. In 12 patients, the response endured for > 90 days, with a median time to recurrence of 5 months (range, 3 to 23 months). The median survival for the SSAg-treated group was 7.9 months (range, 2 to 36 months; 95% confidence interval [CI], 5.9 to 11.4 months), compared to a median survival of 2.5 months (range, 0.1 to 57 months; 95% CI, 1.3 to 3.4 months) for 18 consecutive, unselected patients with MPE from NSCLC (stage IIIb) treated with talc poudrage (p = 0.044). Survival duration of all 14 SSAg-treated cases and 13 talc-poudrage-treated patients with comparable pretreatment KPS (range, 10 to 60; median, 40 and 30, respectively), and distribution (p = 0.5) was 7.9 months (95% CI, 5.9 to 11.4 months) and 2.0 months (95% CI, 0.4 to 2.9 months), respectively (p = 0.0023). Nine of 14 patients treated with SSAg survived > 6 months, 4 patients survived > 9 months, and 3 patients survived > 350 days. One of the patients in the CR group has survived 36 months. None of the 13 talc-treated patients survived > 6 months.

INTERPRETATION

In 14 unselected, consecutive patients with MPE from NSCLC and poor pretreatment performance (median KPS of 40), the intrapleural administration of SSAg was efficacious in resolving the MPE without any clinically important adverse effects. SSAg-treated patients with a median KPS of 40 (range, 10 to 60) had a median survival that exceeded that with talc poudrage, and was comparable to current systemic chemotherapy used in patients with KPS >/= 70 status. SSAg treatment is simple to perform, minimally invasive, and does not require hospital time. It may be an attractive alternative to existing palliative modalities for stage IIIb patients with MPE and poor performance who are not candidates for systemic chemotherapy.

Enhanced expression of lymphotactin by CD8+ T cells is selectively induced by enhancer agonist peptides of tumor-associated antigens

Human tumor-associated antigens (TAAs) are weak immunogens. One strategy for increasing the immunogenicity of TAAs is to generate altered peptide ligands. In the studies reported here, microarray technology has been used to compare gene expression profiles of a human T cell line that was derived from the peripheral blood of a cancer patient vaccinated with a carcinoembryonic antigen (CEA)-based vaccine. We compared the gene expression profiles of this CEA-specific CD8 T cell line when (a) stimulated with the native peptide used to derive this line vs. no peptide, and (b) stimulated with its TCR enhancer agonist epitope vs. no peptide. The results demonstrate that the effect on the T cell line, when stimulated with the agonist peptide, is not an enhanced quantitative expression of the same genes or gene sets induced by the native peptide, but is rather a nearly reciprocal upregulation of different gene sets. The gene for the chemokine lymphotactin, which was overexpressed only in T cells stimulated with the agonist peptide, stood out above all others. This finding was extended using other T cell lines, and another set of agonist and native peptides from another TAA. ELISPOT and ELISA assays for lymphotactin confirmed and extended these findings. These studies suggest a potential role for lymphotactin in the T-cell activation processes and subsequent anti-tumor events.

CD8+αβ+T Cells That Lack Surface CD5 Antigen Expression Are a Major Lymphotactin (XCL1) Source in Peripheral Blood Lymphocytes

To better characterize the cellular source of lymphotactin (XCL1), we compared XCL1 expression in different lymphocyte subsets by real-time PCR. XCL1 was constitutively expressed in both PBMC and CD4(+) cells, but its expression was almost 2 log higher in CD8(+) cells. In vitro activation was associated with a substantial increase in XCL1 expression in both PBMC and CD8(+) cells, but not in CD4(+) lymphocytes. The preferential expression of XCL1 in CD8(+) cells was confirmed by measuring XCL1 production in culture supernatants, and a good correlation was found between figures obtained by real-time PCR and XCL1 contents. XCL1 expression was mostly confined to a CD3(+)CD8(+) subset not expressing CD5, where XCL1 expression equaled that shown by gammadelta(+) T cells. Compared with the CD5(+) counterpart, CD3(+)CD8(+)CD5(-) cells, which did not express CD5 following in vitro activation, showed preferential expression of the alphaalpha form of CD8 and a lower expression of molecules associated with a noncommitted/naive phenotype, such as CD62L. CD3(+)CD8(+)CD5(-) cells also expressed higher levels of the XCL1 receptor; in addition, although not differing from CD3(+)CD8(+)CD5(+) cells in terms of the expression of most alpha- and beta-chemokines, they showed higher expression of CCL3/macrophage inflammatory protein-1alpha. These data show that TCR alphabeta-expressing lymphocytes that lack CD5 expression are a major XCL1 source, and that the contribution to its synthesis by different TCR alphabeta-expressing T cell subsets, namely CD4(+) lymphocytes, is negligible. In addition, they point to the CD3(+)CD8(+)CD5(-) population as a particular T cell subset within the CD8(+) compartment, whose functional properties deserve further attention.

Simian/human immunodeficiency virus(89.6) expressing the chemokine genes MIP-1alpha, RANTES, or lymphotactin

We constructed replication competent, attenuated, nef-deleted SHIV(89.6) that express the rhesus macaque chemokine genes MIP-1alpha, RANTES, or LTN from the nef region. The chemokine inserts were stable during several passages in CEMx174 cells and the viruses grew well in activated rhesus PBMC. Expression of virally encoded MIP-1alpha, RANTES, or LTN was detected in culture fluids from infected HOS CD4(+) CXCR4(+) cells, that were used because they have a low background production of these chemokines. The in vitro growth kinetics of all nef-deleted SHIV(89.6) were slower than the parental strain in both CEMx174 cells and rhesus PBMC. Rhesus macaques were susceptible to SHIV(89.6-MIP-1alpha), SHIV(89.6-RANTES), SHIV(89.6-LTN), and nef-deleted control SHIV(89.6-dLTN) infection via the intrarectal route using standard virus doses, and intact viruses were reisolated from infected animals throughout the interval of acute infection. SHIV expressing the chemokine genes MIP-1alpha, RANTES, or LTN may help determine the in vivo roles for these chemokines in modulating virus replication and disease.