Transcription factor KLF2 regulates homeostatic NK cell proliferation and survival.

Natural killer (NK) cells are innate lymphocytes that recognize and lyse virally infected or transformed cells. This latter property is being pursued in clinics to treat leukemia with the hope that further breakthroughs in NK cell biology can extend treatments to other cancers. At issue is the ability to expand transferred NK cells and prolong their functionality within the context of a tumor. In terms of NK cell expansion and survival, we now report that Kruppel-like factor 2 (KLF2) is a key transcription factor that underpins both of these events. Excision of Klf2 using gene-targeted mouse models promotes spontaneous proliferation of immature NK cells in peripheral tissues, a phenotype that is replicated under ex vivo conditions. Moreover, KLF2 imprints a homeostatic migration pattern on mature NK cells that allows these cells to access IL-15-rich microenvironments. KLF2 accomplishes this feat within the mature NK cell lineage via regulation of a subset of homing receptors that respond to homeostatic ligands while leaving constitutively expressed receptors that recognize inflammatory cytokines unperturbed. Under steady-state conditions, KLF2-deficient NK cells alter their expression of homeostatic homing receptors and subsequently undergo apoptosis due to IL-15 starvation. This novel mechanism has implications regarding NK cell contraction following the termination of immune responses including the possibility that retention of an IL-15 transpresenting support system is key to extending NK cell activity in a tumor environment.

Peripheral tolerance can be modified by altering KLF2-regulated Treg migration.

Tregs are essential for maintaining peripheral tolerance, and thus targeting these cells may aid in the treatment of autoimmunity and cancer by enhancing or reducing suppressive functions, respectively. Before these cells can be harnessed for therapeutic purposes, it is necessary to understand how they maintain tolerance under physiologically relevant conditions. We now report that transcription factor Kruppel-like factor 2 (KLF2) controls naive Treg migration patterns via regulation of homeostatic and inflammatory homing receptors, and that in its absence KLF2-deficient Tregs are unable to migrate efficiently to secondary lymphoid organs (SLOs). Diminished Treg trafficking to SLOs is sufficient to initiate autoimmunity, indicating that SLOs are a primary site for maintaining peripheral tolerance under homeostatic conditions. Disease severity correlates with impaired Treg recruitment to SLOs and, conversely, promotion of Tregs into these tissues can ameliorate autoimmunity. Moreover, stabilizing KLF2 expression within the Treg compartment enhances peripheral tolerance by diverting these suppressive cells from tertiary tissues into SLOs. Taken together, these results demonstrate that peripheral tolerance is enhanced or diminished through modulation of Treg trafficking to SLOs, a process that can be controlled by adjusting KLF2 protein levels.

KLF2 is a rate-limiting transcription factor that can be targeted to enhance regulatory T-cell production.

Regulatory T cells (Tregs) are a specialized subset of CD4(+) T cells that maintain self-tolerance by functionally suppressing autoreactive lymphocytes. The Treg compartment is composed of thymus-derived Tregs (tTregs) and peripheral Tregs (pTregs) that are generated in secondary lymphoid organs after exposure to antigen and specific cytokines, such as TGF-ß. With regard to this latter lineage, pTregs [and their ex vivo generated counterparts, induced Tregs (iTregs)] offer particular therapeutic potential because these cells can be raised against specific antigens to limit autoimmunity. We now report that transcription factor Krüppel-like factor 2 (KLF2) is necessary for the generation of iTregs but not tTregs. Moreover, drugs that limit KLF2 proteolysis during T-cell activation enhance iTreg development. To the authors' knowledge, this study identifies the first transcription factor to distinguish between i/pTreg and tTreg ontogeny and demonstrates that KLF2 is a therapeutic target for the production of regulatory T cells.

The gene expression profile of phosphoantigen-specific human γδ T lymphocytes is a blend of αß T-cell and NK-cell signatures.

Global transcriptional technologies have revolutionised the study of lymphoid cell populations, but human γδ T lymphocytes specific for phosphoantigens remain far less deeply characterised by these methods despite the great therapeutic potential of these cells. Here we analyse the transcriptome of circulating TCRVγ(+) γδ T cells isolated from healthy individuals, and their relation with those from other lymphoid cell subsets. We report that the gene signature of phosphoantigen-specific TCRVγ(+) γδ T cells is a hybrid of those from αß T and NK cells, with more 'NK-cell' genes than αß T cells have and more 'T-cell' genes than NK cells. The expression profile of TCRVγ(+) γδ T cells stimulated with phosphoantigen recapitulates their immediate physiological functions: Th1 cytokine, chemokine and cytotoxic activities reflect their high mitotic activity at later time points and do not indicate antigen-presenting functions. Finally, such hallmarks make the transcriptome of γδ T cells, whether resting or clonally expanding, clearly distinctive from that of NK/T or peripheral T-cell lymphomas of the γδ subtype.

Phosphoantigens overcome human TCRVgamma9+ gammadelta Cell immunosuppression by TGF-beta: relevance for cancer immunotherapy.

Human gammadelta cells expressing TCRVgamma9 are HLA-unrestricted CTLs with high relevance for cancer immunotherapy. Many tumor cell types produce TGF-beta, however, a cytokine strongly immunosuppressive for conventional T CD4, CD8, and NK cells. Whether TGF-beta also inhibits TCRVgamma9+ lymphocytes was unknown. Because phosphoantigens (PAgs), such as bromohydrin pyrophosphate, selectively activate the antitumor functions of TCRVgamma9+ T cells, in this study, we investigated whether TGF-beta modulates these functions. We report that TGF-beta does not block activation of TCRVgamma9+ T cells but inhibits their PAg/IL-2-induced proliferation and maturation into effector cells and finally reduces the cytotoxic activity of these gammadelta T cells when exposed to lymphoma target cells. TGF-beta did not bias their differentiation pattern toward gammadelta Th17 or gammadelta regulatory T cells. Nevertheless, increasing doses of PAg stimulus countered TGF-beta inhibition. So, although TGF-beta impairs TCRVgamma9+ gammadelta cells like other cytolytic lymphocytes, PAg alone or combined to therapeutic mAb has the ability to bypass its immunosuppressive activity.

Bromohydrin pyrophosphate enhances antibody-dependent cell-mediated cytotoxicity induced by therapeutic antibodies.

In human blood, 1% to 5% of lymphocytes are gammadelta T cells; they mostly express the gammadelta T-cell receptor (TCR)Vgamma9, recognize nonpeptide phosphoantigens (PAgs) produced by microbes and tumor cells, and mediate different modes of lytic activities directed against tumor target cells. Antibody-dependent cell-mediated cytotoxicity (ADCC) mediated by cytolytic lymphoid cells is essential for the clinical activity of anticancer monoclonal antibodies (mAbs), but whether PAgs affect ADCC by gammadelta T cells is unknown. Here we report that, in association with the CD20(+)-specific mAb rituximab (RTX), the synthetic PAg bromohydrin pyrophosphate (BrHPP) increased TCRVgamma9(+) cell binding to CD20(+) lymphoma cells in vitro. This combination activated phospho-ZAP70 and phospho-ERK1/2 signaling in TCRVgamma9(+) cells and strongly enhanced their ADCC activity. We obtained similar results with BrHPP in the context of the mAbs alemtuzumab and trastuzumab. Furthermore, BrHPP enhanced RTX-mediated depletion of CD20(+) cells in vitro from peripheral blood mononuclear cells of healthy subjects and enhanced ADCC by gammadelta T cells from patients with chronic lymphocytic leukemia. In cynomolgus macaques, a regimen combining RTX, BrHPP, and IL2 activated TCRVgamma9(+) lymphocytes and enhanced B-cell depletion from blood and lymph nodes. Thus, the combination with BrHPP PAg is able to improve the efficacy of cancer immunotherapy by therapeutic mAbs.

A tuberculosis vaccine based on phosphoantigens and fusion proteins induces distinct gammadelta and alphabeta T cell responses in primates.

Phosphoantigens are mycobacterial non-peptide antigens that might enhance the immunogenicity of current subunit candidate vaccines for tuberculosis. However, their testing requires monkeys, the only animal models suitable for gammadelta T cell responses to mycobacteria. Thus here, the immunogenicity of 6-kDa early secretory antigenic target-mycolyl transferase complex antigen 85B (ESAT-6-Ag85B) (H-1 hybrid) fusion protein associated or not to a synthetic phosphoantigen was compared by a prime-boost regimen of two groups of eight cynomolgus. Although phosphoantigen activated immediately a strong release of systemic Th1 cytokines (IL-2, IL-6, IFN-gamma, TNF-alpha), it further anergized blood gammadelta T lymphocytes selectively. By contrast, the hybrid H-1 induced only memory alphabeta T cell responses, regardless of phosphoantigen. These latter essentially comprised cytotoxic T lymphocytes specific for Ag85B (on average + 430 cells/million PBMC) and few IFN-gamma-secreting cells (+ 40 cells/million PBMC, equally specific for ESAT-6 and for Ag85B). Hence, in macaques, a prime-boost with the H-1/phosphoantigen subunit combination induces two waves of immune responses, successively by gammadelta T and alphabeta T lymphocytes.