Superantigen enhanced protection against a weak tumor-specific melanoma antigen: implications for prophylactic vaccination against cancer

Endogenous HLA-DQ8αβ programs superantigens (SEG/SEI) to silence toxicity and unleash a tumoricidal network with long-term melanoma survival

Background

As the most powerful T cell agonists known, superantigens (SAgs) have enormous potential for cancer immunotherapy. Their development has languished due to high incidence (60%–80%) of seroreactive neutralizing antibodies in humans and tumor necrosis factor-α (TNFα)-mediated cardiopulmonary toxicity. Such toxicity has narrowed their therapeutic index while neutralizing antibodies have nullified their therapeutic effects.

Methods

Female HLA-DQ8 (DQA*0301/DQB*0302) tg mice expressing the human major histocompatibility complex II (MHCII) HLA-DQ8 allele on a high proportion of PBL, spleen and lymph node cells were used. In the established tumor model, staphylococcal enterotoxin G and staphylococcal enterotoxin I (SEG/ SEI) (50 µg each) were injected on days 6 and 9 following tumor inoculation. Lymphoid, myeloid cells and tumor cell digests from tumor tissue were assayed using flow cytometry or quantitated using a cytometric bead array. Tumor density, necrotic and viable areas were quantitated using the ImageJ software.

Results

In a discovery-driven effort to address these problems we introduce a heretofore unrecognized binary complex comprizing SEG/SEI SAgs linked to the endogenous human MHCII HLA-DQ8 allele in humanized mice. By contrast to staphylococcal enterotoxin A (SEA) and staphylococcal enterotoxin B (SEB) deployed previously in clinical trials, SEG and SEI does not exhibit neutralizing antibodies in humans or TNFα-mediated toxicity in humanized HLA-DQ8 mice. In the latter model wherein SAg behavior is known to be ‘human-like’, SEG/SEI induced a powerful tumoricidal response and long-term survival against established melanoma in 82% of mice. Other SAgs deployed in the same model displayed toxic shock. Initially, HLA-DQ8 mediated melanoma antigen priming, after which SEG/SEI unleashed a broad CD4+ and CD8+ antitumor network marked by expansion of melanoma reactive T cells and interferon-γ (IFNy) in the tumor microenvironment (TME). SEG/SEI further initiated chemotactic recruitment of tumor reactive T cells to the TME converting the tumor from ‘cold’ to a ‘hot’. Long-term survivors displayed remarkable resistance to parental tumor rechallenge along with the appearance of tumor specific memory and tumor reactive T memory cells.

Conclusions

Collectively, these findings show for the first time that the SEG/SEI-(HLA-DQ8) empowers priming, expansion and recruitment of a population of tumor reactive T cells culminating in tumor specific memory and long-term survival devoid of toxicity. These properties distinguish SEG/SEI from other SAgs used previously in human tumor immunotherapy. Consolidation of these principles within the SEG/SEI-(HLA-DQ8) complex constitutes a conceptually new therapeutic weapon with compelling translational potential.

The Role of Bacterial Superantigens in the Immune Response: From Biology to Cancer Treatment

Aims:

Encouraging results have been indicated preclinically and in patients using the bacterial superantigen. This review article intends to summarize the role of the superantigens that have been recently used in the treatment of cancer. In addition, the vector systems, including lentiviral vectors, adeno-associated vector systems and retroviral vectors that are increasingly being used in basic and applied research, were discussed. Most importantly, the new CRISPR technique has also been discussed in this literature review.

Discussion:

More successful therapies can be achieved by manipulating bacterial vector systems through incorporating genes related to the superantigens and cytokines. The products of SAg and cytokine genes contribute to the strong stimulation of the immune system against tumor cells. They bind to MHC II molecules as well as the V beta regions of TCR and lead to the production of IL2 and other cytokines, the activation of antigen-presenting cells and T lymphocytes. Additionally, superantigens can be used to eradicate tumor cells. Better results in cancer treatment can be achieved by transferring superantigen genes and subsequent strong immune stimulation along with other cancer immunotherapy agents.

Conclusion:

Superantigens induce the proliferation of T lymphocytes and antigen-presenting cells by binding to MHCII molecules and V beta regions in T cell receptors. Therefore, the presentation of tumor cell antigens is increased. Additionally, the production of important cytokines by T cells and APCs contributes to the stimulation of immune response against tumor cells. The manipulation of bacterial vector systems through incorporating genesrelated to SAgs and other immune response factors is a good strategy for the immune system stimulating and eradicating tumor cells along with other immunotherapy agents.

Staphylococcal Enterotoxin Superantigens Induce Prophylactic Antiviral Activity Against Encephalomyocarditis Virus In Vivo and In Vitro

The staphylococcal enterotoxins (SEs) are classified as superantigens due to their potent stimulation of the immune system resulting in T cell activation and prodigious cytokine production and toxicity. This study examined the ability of superantigens to induce prophylactic antiviral activity in vivo and in vitro and evaluated potential superantigen mimetic peptides. Prophylactic treatment of mice in vivo with intraperitoneal injections of SE superantigens SEA and SEB (both at 20 μg/day for 3 days) prevented encephalomyocarditis virus (EMCV)-induced lethality in 100% and 80% of mice, respectively, as compared with control saline-treated groups in which EMCV was lethal to all mice. Furthermore, SEA (2 μg/mL) and SEB (1 μg/mL) induced antiviral activity in mouse splenocytes to produce an antiviral factor since their supernatant prevented EMCV lysis of L929 cells in tissue culture. It was found that superantigens do not directly prevent EMCV infection, but rather indirectly through inducing interferon gamma (IFNγ) production in cells as the antiviral factor. Evaluation of various superantigen mimetic peptides showed that one peptide (SEA3) had superantigen-like activity by inducing IFNγ production in cells but without the cellular proliferation, as associated with superantigens. However, the induction of IFNγ activation by the SEA3 peptide was not as pronounced, and took a much higher peptide concentration, when compared with the parent superantigen. If the negative side effects of superantigens can be eliminated, their beneficial properties can be harnessed for prophylactic treatment of viral infections and other pathologies requiring a robust immune response.

Bone marrow - mesenchymal stem cells impact on the U937 cells in the presence of staphylococcal enterotoxin B (SEB)

The growing resistance against conventional chemotherapy in acute myeloid leukemia (AML) is a noticeable clinical concern. Therefore, many researchers are looking for novel substances to overcome drug resistance in cancer. Staphylococcal enterotoxin B (SEB) is a superantigen (SAg) and a promising compound which has lethal effects on malignant cells. In this unprecedented study, SEB was used against U937 cells in a co-culture system in the presence of human bone marrow-mesenchymal stem cells (hBM-MSCs). The effects of hBM-MSCs on the proliferation and survival of U937 cell line with SEB was assessed using MTT assay and AnnexinV/PI flowcytometry, respectively. Moreover, the expression of IL-6, IL-10, TGF-β, and inhibitor of nuclear factor kappa-B kinase (IKKb) was evaluated by real-time PCR technique. The same experiments were also carried out using hBM-MSCs-conditioned medium (hBM-MSCs-CM). The results showed that SEB reduced the proliferation and survival of U937 cell line, but hBM-MSCs or hBM-MSCs-CM suppressed the effects of SEB. Furthermore, real-timePCR demonstrated that SEB could decrease the expression of IL-6, IL-10, and TGF-β in hBM-MSCs (P < .05), while the production of IKKb was increased in comparison with the control group. These findings help us to have a broader understanding ofthe usage of SEB in the treatment of haematological malignancies, especially if it is targeted against hBM-MSCs to disrupt their supportive effects on malignant cells.

A commercially available preparation of Staphylococcus aureus bio-products potently inhibits tumour growth in a murine model of mesothelioma

BACKGROUND AND OBJECTIVE

Mesothelioma is an incurable cancer with a rising global incidence. Intrapleural delivery of a commercially available compound made up of proteins produced by Staphylococcus aureus has been used clinically to induce pleurodesis. We investigate if this bacterial compound has anti-tumoural activities against pleural malignancies, in addition to its pleurodesing effect.

METHODS

The effects of the treatment on mesothelioma cells were evaluated in vitro and further tested in two validated murine models.

RESULTS

This S. aureus bio-product mixture effectively kills mesothelioma cells and induces the release of interleukin (IL)-8, monocyte chemotactic protein (MCP)-1 and vascular endothelial growth factor from primary human mesothelial cells but not malignant pleural mesothelioma cells in vitro. Intratumoural delivery of the treatment in BALB/c mice induced tumour necrosis and local activation of T cells. Tumour growth was significantly inhibited in the treatment group during and after the treatment period (size of tumour 58.8 ± 10.3 mm(2) vs 118.3 ± 6.7 mm(2) from saline controls at day 23, n = 9-12 per group), P < 0.001. Tumour growth resumed on cessation of treatment, confirming the inhibition was treatment related. Treatment benefits were further validated in an orthotopic peritoneal model of mesothelioma and the compound significantly reduced the mesothelioma load (P < 0.05 vs saline controls). Mice in the treatment group had a significant increase in the percentage of activated CD4(+) and CD8(+) T cells in tumour-draining lymph nodes. No histological side-effects were observed with the treatment.

CONCLUSIONS

This proof-of-principle study demonstrates promising antitumoural activity of a commercially available compound of S. aureus bio-products against mesothelioma.

Evaluation of a recombinant double mutant of staphylococcal enterotoxin B (SEB-H32Q/K173E) with enhanced antitumor activity effects and decreased pyrexia

BACKGROUND

Immunotherapy has been used to improve patient immune function, inhibit tumor growth and has become a highly promising method of cancer treatment. Highly agglutinative staphylococcin (HAS), a mixture of Staphylococcus aureus culture filtrates, which include staphylococcal enterotoxin (SE) C as the active ingredient, has been used clinically as an immunomodifier in the treatment of a number of tumors for many years. However, the use of HAS has been associated with some unavoidable side-effects such as fever. Previous studies have shown that SEB stimulates a more potent activation of T lymphocytes than SEC3, and mutations of the histidine residues eliminated the toxicity of SEB. SE mutants with decreased side-effects and/or more potent antitumor activities are required.

METHODOLOGY/PRINCIPAL FINDINGS

We built a structural model of the MHC II-SEB-TCR complex and found that a mutation of SEB at Lys173 might decrease the repulsion force between the SEB-TCR, which would facilitate their interaction. From the above results, we designed SEB-H32Q/K173E (mSEB). Analysis of in vitro stimulation of the proliferation of human peripheral blood mononuclear cells (PBMCs), IFN-γ secretion and inhibition of the growth of various tumor cell lines demonstrated that mSEB exhibited higher antitumor activity compared with wild-type SEB (wtSEB). Notably, mSEB inhibited the growth of various tumors at an extremely low concentration with little cytotoxicity against normal cells. Three animal tumor models (C57BL/6 mouse, New Zealand rabbit and a humanized NOD/SCID mouse) were used to evaluate the in vivo immunotherapeutic effects. Compared with wtSEB, mSEB significantly enhanced antitumor effect in more than one animal model with reduced pyrexia toxicity and prolonged the survival of tumor-bearing mice.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that SEB-H32Q/K173E retains superantigen (SAg) characteristics and enhances the host immune response to neoplastic diseases while reducing associated pyrogenic toxicity.

Increased T-cell stimulating activity by mutated SEC2 correlates with its improved antitumour potency

AIMS

To investigate the improved antitumour activity of SAM-3 compared with recombinant staphylococcal enterotoxins C2 (rSEC2).

METHODS AND RESULTS

Methylthiazol tetrazolium and flow cytometry assays showed that the antitumour activity of SAM-3 in vivo was improved because of enhanced T-cell stimulating potency, resulting in massive activation of T cells, particularly CD4(+) and CD8(+) T cells, and subsequent cytokine release. Quantitative real-time PCR assay showed that despite similar Vβ specificities induced by rSEC2 and SAM-3, the quantities of activated T cells bearing specific Vβin vitro were different.

CONCLUSIONS

The results strongly suggested that the increased SAM-3-T-cell receptor (TCR) binding affinity contributed to massive T-cell activation and cytokine release, substantially amplifying antitumour immune response in vivo.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provided evidence for the mechanism of SAM-3 antitumour activity improvement compared with rSEC2. Results indicated that SAM-3 could be used as a potent powerful candidate agent for tumour treatment in clinics.

Enhanced anti-tumor immunity by superantigen-pulsed dendritic cells

Staphylococcal enterotoxins A (SEA) and B (SEB) are classical models of superantigens (SAg), which induce potent T-cell-stimulating activity by forming complexes with MHC class II molecules on antigen-presenting cells. This large-scale activation of T-cells is accompanied by increased production of cytokines such as interferon-γ (IFN-γ). Additionally, as we previously reported, IFN-γ-producing CD8(+) T cells act as "helper cells," supporting the ability of dendritic cells to produce interleukin-12 (IL-12)p70. Here, we show that DC pulsed with SAg promote the enhancement of anti-tumor immunity. Murine bone marrow-derived dendritic cells (DC) were pulsed with OVA(257-264) (SIINFEKL), which is an H-2Kb target epitope of EG7 [ovalbumin (OVA)-expressing EL4] cell lines, in the presence of SEA and SEB and were subcutaneously injected into naïve C57BL/6 mice. SAg plus OVA(257-264)-pulsed DC vaccine strongly enhanced peptide-specific CD8(+) T cells exhibiting OVA(257-264)-specific cytotoxic activity and IFN-γ production, leading to the induction of protective immunity against EG7 tumors. Furthermore, cyclophosphamide (CY) added to SAg plus tumor-antigens (OVA(257-264), tumor lysate, or TRP-2) pulsed DC immunization markedly enhanced tumor-specific T-cell expansion and had a significant therapeutic effect against various tumors (EG7, 2LL, and B16). Superantigens are potential candidates for enhancing tumor immunity in DC vaccines.

Superantigens increase the survival of mice bearing T cell lymphomas by inducing apoptosis of neoplastic cells

Superantigens bind to major histocompatibility complex class II molecules and interact with T cells expressing a particular T cell receptor Vβ inducing a strong proliferation/deletion response of the superantigen-reactive T cells. However, there have been no attempts to investigate the ability of Sags to induce apoptosis in neoplastic T cells by signaling through the Vβ region of their TCR. In the present study we show that bacterial and MMTV-encoded superantigens induce the apoptosis of AKR/J cognate lymphoma T cells both in vitro and in vivo. The Fas-Fas-L pathway was shown to be involved in the apoptosis of lymphoma T cells induced by bacterial superantigens. In vivo exposure to bacterial superantigens was able to improve the survival of lymphoma bearing mice. Moreover, the permanent expression of a retroviral encoded superantigen induced the complete remission of an aggressive lymphoma in a high percentage of mice. The possibility of a therapeutic use of superantigens in lymphoma/leukemia T cell malignancies is discussed.

In vivo and in vitro antitumor effects of a staphylococcal enterotoxin A mutant (SEA-H61D)

In this study, we evaluated SEA-H61D, a staphylococcal enterotoxin A mutant without emetic activity, as an antitumor agent in vitro and in vivo. It showed that SEA-H61D could significantly inhibit the growth of many cancer cell lines in vitro at very low concentrations by activating human peripheral blood mononuclear cells (PBMCs). CD4+ and CD8+ T lymphocytes could be activated at a dose between 125 and 500 μg/kg. Systemic administration of SEA-H61D in vivo significantly inhibited tumor growth, with the treated group undergoing tumor necrosis and showing a strong infiltration of lymphocytes to the tumor area.

In vivo induction of necrosis in mice fibrosarcoma via intravenous injection of type B staphylococcal enterotoxin

The bacterial superantigen staphylococcal enterotoxin B (SEB) is a potent inducer of cytotoxic T-cell activity and cytokine production in vivo. We investigated the possibility of the therapeutic application of SEB in patients with fibrosarcoma. The anti-tumor effect of SEB in mice with inoculated fibrosarcoma (WEHI-164) was examined by intravenous (IV) and intratumoral (IT) injection and the sizes of the inoculated tumors, IFN-gamma production, and CD4+/CD8+ T cell infiltration were determined. The inoculated tumors were also examined histologically. In the mice in the IV-injected group, a significant reduction (P < 0.02) of tumor size was observed in comparison with mice in the IT-injected and control groups. Furthermore, the mice in the IV-injected group showed significantly higher levels of IFN-gamma (P < 0.009) and CD4+/CD8+ T cell infiltration when compared with the other groups (P < 0.02). A significantly higher frequency of necrosis in tumor tissues was also observed in mice in the IV-injected group (P < 0.05). Our present findings suggest that tumor cell death is caused by increased cytotoxic T-cell activity and cytokine levels in response to the IV injection of SEB and that SEB may be a good option for use as a novel therapy in patients with fibrosarcoma.

Polyethylenimine-mediated in vivo gene transfer of a transmembrane superantigen fusion construct inhibits B16 murine melanoma growth

Immunotherapy has been proposed as a therapeutic strategy in advanced-stage melanomas in which other therapeutic options have little effect. The Staphylococcus enterotoxin A (SEA) has been used to stimulate an antitumoral immune response but its use is hampered by severe systemic side effects. Here, we show that SEA can be targeted to melanoma cells to limit these side effects. More specifically, we used a nonviral vector, the cationic polymer, polyethylenimine (PEI), to express a transmembrane SEA fusion construct (pSEA-TM) in B16F10-induced subcutaneous melanoma in mice. The efficacy of this in vivo transfection was enhanced by concomitant infusion of epinephrine to induce local vasoconstriction. In these conditions, repeated injections of pSEA-TM/PEI complexes elicited a significant response, as evidenced by tumor growth inhibition, without systemic adverse effects. T cell infiltration of the tumors, together with positive lymphocyte proliferation tests, suggested local and systemic immune responses. Altogether, PEI-mediated targeting of SEA to melanoma tumor cells in vivo efficiently stimulates the antitumor immune response without inducing the side effects observed with systemic administration of SEA.

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.

Preclinical evaluation of superantigen (staphylococcal enterotoxin B) in the intravesical immunotherapy of superficial bladder cancer

Superantigens are potent activators of T lymphocytes; therefore, their characteristics can be exploited in diseases where immunomodulation is known to be effective. In this study, we evaluated a new approach for the intravesical therapy of superficial bladder cancer. We investigated in coculture experiments if staphylococcal enterotoxin B (SEB)-activated PBMCs are able to induce apoptosis in human transitional cell carcinoma (TCC) cells. Additionally, we tested the toxicity and efficacy of SEB dissolved in NaCl 0.9% administered intravesically once weekly for 6 weeks in a rat bladder cancer model. To validate the coculture in vitro findings, we evaluated tumor stage, grade, apoptotic cells in the urothelium and stroma of the bladder and infiltration of the bladder wall by lymphocytes, macrophages and mononuclear cells. Coculture experiments revealed that SEB-activated PBMCs are able to kill TCC cells by inducing apoptosis. The intravesical toxicity study with a maximum dose of 100 microg/ml SEB demonstrated no side effects. In the intravesically SEB-treated animals (10 microg/ml), only 3 tumors remained vs. 15 persisting tumors in the control group. The remaining tumors of the therapy group showed a significant amount of apoptosis and granulocytes, mainly in the urothelium, whereas no relevant apoptosis or infiltration of the bladder with lymphocytes or macrophages was found in the control group. These preclinical findings suggest that SEB might be an interesting candidate for further clinical evaluation.

Depletion of activated Vbeta8+ T cells disrupts bispecific antibody directed antitumor immunity

INTRODUCTION

Activation of Vbeta8+ T cells with superantigen staphylococcal enterotoxin B (SEB) and use of an antitumor, anti-CD3 bispecific antibody (BsAb) leads to tumor protective immunity. We hypothesize that Vbeta8+ T-cell activation in combination with BsAb is crucial for tumor protective immunity in this model.

METHODS

Adolescent C3H/HeN mice were intravenously injected with syngeneic CL62 melanoma to establish pulmonary metastasis. Three days after establishing pulmonary metastasis, predominantly Vbeta8+ T cells are activated with 50 mug of intraperitoneal superantigen SEB. T cells were depleted at different time points in relation to SEB administration to assess the effect on protective immunity against a second tumor challenge.

RESULTS

Protective immunity is significantly (P < 0.008) decreased when Vbeta8+ depletion occurs 6 h after SEB injection, as growth of rechallenged CL62 melanoma occurred in 43%. Protective immunity is present at all other time points when mice survive Vbeta8+ T-cell depletion. Survival of animals treated with SEB/BsAb (82%) is significantly better (P < 0.002) than with SEB alone (60%) or nontreated control (0%). Survival when Vbeta8+ T-cell depletion occurred at 6 h and 48 h post-SEB is 72% and 77%, respectfully, and is statistically indistinguishable (P < 0.232 and P < 0.602). If T-cell depletion was conducted before SEB administration, however, the combination of SEB and BsAb did not result in significant protective immunity. T-cell depletion before the use of SEB alone, without BsAb, failed to result in significant protective immunity.

CONCLUSIONS

Depletion of Vbeta8+ T cells 6 h after activation disrupts the development of protective immunity.

Potent antitumor effect elicited by superantigen-linked tumor cells transduced with heat shock protein 70 gene

Heat shock proteins (HSP) induce antitumor-specific immunity via a unique mechanism, but HSP alone fails to produce a satisfactory antitumor efficacy. We considered that the potent immune-activation of superantigen (SAg) might assist HSP to elicit a strong tumor-antigen-specific immunity. We initially prepared B16 melanoma cells linked to SAg SEA via a fusion protein with a transmembrane sequence (TM), and demonstrated that SEA thus anchored on the tumor cell surface could elicit strong antitumor immunity. We then prepared cells transduced with an inducible heat shock protein 70 (HSP70) gene, and bearing SEA-TM fusion protein on the cell surface, and used these cells as a dual-modified vaccine. In this study, either in a therapeutic setting or in a pre-immune model, the SEA-anchored vaccine or the HSP70 gene-modified vaccine induced marked tumor suppression, prolonged survival, augmented lymphocyte proliferation and higher NK and CTL activity in C57BL/6 mice compared with their controls (P < 0.01), though they were less effective than the dual-modified vaccine. Among these vaccines, the dual-modified vaccine showed the best therapeutic efficacy in B16 melanoma-bearing mice and gave the greatest protection against wild-type B16 melanoma challenge. The results indicated that the dual-modified vaccine could induce a potent tumor-antigen-specific immune response in addition to an increase of non-specific immunity. This study offers a novel approach to bridging specific and non-specific immunity for cancer therapy.

Superantigen enhancement of specific immunity: antibody production and signaling pathways

Superantigens are microbial proteins that induce massive activation, proliferation, and cytokine production by CD4+ T cells via specific Vbeta elements on the TCR. In this study we examine superantigen enhancement of Ag-specific CD4+ T cell activity for humoral B cell responses to T-dependent Ags BSA and HIV gp120 envelope, type I T-independent Ag LPS, and type II T-independent Ag pneumococcal polysaccharides. Injection of BSA followed by a combination of superantigens staphylococcal enterotoxin A and staphylococcal enterotoxin B (SEB) 7 days later enhanced the anti-BSA Ab response in mice approximately 4-fold as compared with mice given BSA alone. The anti-gp120 response was enhanced approximately 3-fold by superantigens. The type II T-independent Ag pneumococcal polysaccharide response was enhanced approximately 2.3-fold by superantigens, whereas no effect was observed on the response to the type I T-independent Ag LPS. The superantigen effect was completely blocked by the CD4+ T cell inhibitory cytokine IL-10. SEB-stimulated human CD4+ T cells were examined to determine the role of the mitogen-activated protein (MAP) kinase signal transduction pathway in superantigen activation of T cells. Inhibitors of the mitogen pathway of MAP kinase blocked SEB-induced proliferation and IFN-gamma production, while an inhibitor of the p38 stress pathway had no effect. Consistent with this, SEB activated extracellular signal-regulated kinase/MAP kinase as well as MAP kinase-interacting kinase, a kinase that phosphorylates eIF4E, which is an important component of the eukaryotic protein synthesis initiation complex. Both kinases were inhibited by IL-10. Thus, superantigens enhance humoral immunity via Ag-specific CD4+ T cells involving the stress-independent pathway of MAP kinase.