Emerging phagocytosis checkpoints in cancer immunotherapy

Cancer immunotherapy, mainly including immune checkpoints-targeted therapy and the adoptive transfer of engineered immune cells, has revolutionized the oncology landscape as it utilizes patients' own immune systems in combating the cancer cells. Cancer cells escape immune surveillance by hijacking the corresponding inhibitory pathways via overexpressing checkpoint genes. Phagocytosis checkpoints, such as CD47, CD24, MHC-I, PD-L1, STC-1 and GD2, have emerged as essential checkpoints for cancer immunotherapy by functioning as "don't eat me" signals or interacting with "eat me" signals to suppress immune responses. Phagocytosis checkpoints link innate immunity and adaptive immunity in cancer immunotherapy. Genetic ablation of these phagocytosis checkpoints, as well as blockade of their signaling pathways, robustly augments phagocytosis and reduces tumor size. Among all phagocytosis checkpoints, CD47 is the most thoroughly studied and has emerged as a rising star among targets for cancer treatment. CD47-targeting antibodies and inhibitors have been investigated in various preclinical and clinical trials. However, anemia and thrombocytopenia appear to be formidable challenges since CD47 is ubiquitously expressed on erythrocytes. Here, we review the reported phagocytosis checkpoints by discussing their mechanisms and functions in cancer immunotherapy, highlight clinical progress in targeting these checkpoints and discuss challenges and potential solutions to smooth the way for combination immunotherapeutic strategies that involve both innate and adaptive immune responses.

Review: PD-L1 as an emerging target in the treatment and prevention of keratinocytic skin cancer

Recent advances in the understanding and targeting of immune checkpoints have led to great progress in immune therapies against many forms of cancer. While many types of immune checkpoints are currently targeted in the clinic, this review will focus on recent research implicating the programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) axis as an emerging focus for the treatment of keratinocytic tumors. PD-L1 is of particular interest in nonmelanoma skin cancer (NMSC), as it is not only upregulated in these tumors but is stimulated by environmental ultraviolet exposure. This response may also make PD-L1 an excellent target for photochemoprevention using topically applied small molecule inhibitors. Here, we summarize recent investigations on PD-L1 expression and clinically relevant immune checkpoint inhibitor treatment in cutaneous squamous cell carcinoma, basal cell carcinoma, and head and neck squamous cell carcinoma, as well as small molecule agents targeting PD-L1 that may be useful for clinical development aiming at treatment or prevention of NMSC.

B7-H4 is a potential diagnostic and prognostic biomarker in colorectal cancer and correlates with the epithelial-mesenchymal transition

Background

As a negative co-stimulatory molecule of the B7 family, B7-H4 has recently attracted increased attention. However, the clinical value of B7-H4 in colorectal cancer (CRC) remains controversial and requires further investigation. This study aimed to investigate the role of B7-H4 in the clinical diagnosis and survival prognosis of CRC.

Methods

The relationships between B7-H4 expression, immune cell infiltration, epithelial-mesenchymal transition (EMT), clinicopathological features, and survival prognosis were determined through the TCGA database and verified in a large CRC cohort (n = 1118).

Results

The results showed the level of B7-H4 mRNA expression was significantly increased in the CRC tumor tissues compared with normal tissues (P <  0.001). Immunohistochemistry showed that B7-H4 protein expression was also up-regulated in CRC. The positive rate of B7-H4 in CRC tumor tissues was 76.38%, which was significantly higher than that in non-tumor tissues (P <  0.001). Overexpression of B7-H4 was positively correlated with lymph node metastasis, advanced TNM stage, and poor tumor differentiation (P = 0.012; 0.009; 0.014). Prognostic analysis showed high B7-H4 expression was associated with significantly shorter OS. Multivariate analysis demonstrated the risk of death in CRC patients with high B7-H4 expression is 1.487 times that of low B7-H4 expression. In addition, B7-H4 expression was negatively correlated with the epithelial marker E-cadherin (P <  0.001) and positively correlated with the mesenchymal marker vimentin (P <  0.001) in CRC tissues. However, B7-H4 expression was not associated with the immunosuppressive microenvironment in CRC.

Conclusion

B7-H4 may represent a potential biomarker for the diagnosis and prognosis of CRC and enhance CRC invasion by promoting EMT.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-10159-5.

Immunohistochemical Study of the PD-1/PD-L1 Pathway in Cutaneous Lupus Erythematosus

The pathomechanism of various autoimmune diseases is known to be associated with the altered function of programmed cell death 1/programmed cell death ligand 1 (PD-1/PD-L1) axis. We aimed to investigate the role of this pathway and inflammatory cell markers in subtypes of cutaneous lupus erythematosus (CLE): discoid lupus erythematosus (DLE), subacute CLE (SCLE) and toxic epidermal necrolysis (TEN)-like lupus, a hyperacute form of acute CLE (ACLE). Ten skin biopsy samples from 9 patients were analyzed with immunohistochemistry regarding the following markers: CD3, CD4, CD8, Granzyme B, CD123, CD163, PD-1, PD-L1. Our group consisted of 4 SCLE (2 idiopathic (I-SCLE) and 2 PD-1 inhibitor-induced (DI-SCLE)), 4 DLE and 1 TEN-like lupus cases. From the latter patient two consecutive biopsies were obtained 1 week apart. Marker expression patterns were compared through descriptive analysis. Higher median keratinocyte (KC) PD-L1 expression was observed in the SCLE group compared to the DLE group (65% and 5%, respectively). Medians of dermal CD4, Granzyme B (GB), PD-1 positive cell numbers and GB+/CD8⁺ ratio were higher in the DLE group than in the SCLE group. The I-SCLE and DI-SCLE cases showed many similarities, however KC PD-L1 expression and dermal GB positive cell number was higher in the former. The consecutive samples of the TEN-like lupus patient showed an increase by time within the number of infiltrating GB+ cytotoxic T-cells and KC PD-L1 expression (from 22 to 43 and 30%–70%, respectively). Alterations of the PD-1/PD-L1 axis seems to play a role in the pathogenesis of CLE.

Immunosuppressive Properties of Epidermal Keratinocytes Differ According to Their Immaturity Status

Preservation of a functional keratinocyte stem cell pool is essential to ensure the long-term maintenance of epidermis integrity, through continuous physiological renewal and regeneration in case of injury. Protecting stem cells from inflammation and immune reactions is thus a critical issue that needs to be explored. Here, we show that the immature CD49f^high precursor cell fraction from interfollicular epidermis keratinocytes, comprising stem cells and progenitors, is able to inhibit CD4⁺ T-cell proliferation. Of note, both the stem cell-enriched CD49f^high/EGFR^low subpopulation and the less immature CD49f^high/EGFR^high progenitors ensure this effect. Moreover, we show that HLA-G and PD-L1 immune checkpoints are overexpressed in CD49f^high precursors, as compared to CD49f^low differentiated keratinocytes. This potency may limit immune reactions against immature precursors including stem cells, and protect them from exacerbated inflammation. Further exploring this correlation between immuno-modulation and immaturity may open perspectives in allogenic cell therapies.

Skin Immunity and Tolerance: Focus on Epidermal Keratinocytes Expressing HLA-G

Although the role of epidermal cells in skin regeneration has been extensively documented, their functions in immunity and tolerance mechanisms are largely underestimated. The aim of the present review was to outline the state of knowledge on resident immune cells of hematopoietic origin hosted in the epidermis, and then to focus on the involvement of keratinocytes in the complex skin immune networks acting in homeostasis and regeneration conditions. Based on this knowledge, the mechanisms of immune tolerance are reviewed. In particular, strategies based on immunosuppression mediated by HLA-G are highlighted, as recent advances in this field open up perspectives in epidermis-substitute bioengineering for temporary and permanent skin replacement strategies.

Antigen presentation by lung epithelial cells directs CD4+ TRM cell function and regulates barrier immunity

Barrier tissues are populated by functionally plastic CD4+ resident memory T (TRM) cells. Whether the barrier epithelium regulates CD4+ TRM cell locations, plasticity and activities remains unclear. Here we report that lung epithelial cells, including distinct surfactant protein C (SPC)lowMHChigh epithelial cells, function as anatomically-segregated and temporally-dynamic antigen presenting cells. In vivo ablation of lung epithelial MHC-II results in altered localization of CD4+ TRM cells. Recurrent encounters with cognate antigen in the absence of epithelial MHC-II leads CD4+ TRM cells to co-express several classically antagonistic lineage-defining transcription factors, changes their cytokine profiles, and results in dysregulated barrier immunity. In addition, lung epithelial MHC-II is needed for surface expression of PD-L1, which engages its ligand PD-1 to constrain lung CD4+ TRM cell phenotypes. Thus, we establish epithelial antigen presentation as a critical regulator of CD4+ TRM cell function and identify epithelial-CD4+ TRM cell immune interactions as core elements of barrier immunity.

Human Keratinocytes Inhibit CD4+ T-Cell Proliferation through TGFB1 Secretion and Surface Expression of HLA-G1 and PD-L1 Immune Checkpoints

Human skin protects the body against infection and injury. This protection involves immune and epithelial cells, but their interactions remain largely unknown. Here, we show that cultured epidermal keratinocytes inhibit allogenic CD4⁺ T-cell proliferation under both normal and inflammatory conditions. Inhibition occurs through the secretion of soluble factors, including TGFB1 and the cell-surface expression of HLA-G1 and PD-L1 immune checkpoints. For the first time, we here describe the expression of the HLA-G1 protein in healthy human skin and its role in keratinocyte-driven tissue immunomodulation. The overexpression of HLA-G1 with an inducible vector increased the immunosuppressive properties of keratinocytes, opening up perspectives for their use in allogeneic settings for cell therapy.

Contact dermatitis

Contact dermatitis (CD) is among the most common inflammatory dermatological conditions and includes allergic CD, photoallergic CD, irritant CD, photoirritant CD (also called phototoxic CD) and protein CD. Occupational CD can be of any type and is the most prevalent occupational skin disease. Each CD type is characterized by different immunological mechanisms and/or requisite exposures. Clinical manifestations of CD vary widely and multiple subtypes may occur simultaneously. The diagnosis relies on clinical presentation, thorough exposure assessment and evaluation with techniques such as patch testing and skin-prick testing. Management is based on patient education, avoidance strategies of specific substances, and topical treatments; in severe or recalcitrant cases, which can negatively affect the quality of life of patients, systemic medications may be needed.

Overexpression of PD-L1 in gingival basal keratinocytes reduces periodontal inflammation in a ligature-induced periodontitis model

Background

The immune checkpoint programmed cell death 1 (PD‐1): PD‐1 ligand 1 (PD‐L1) pathway plays a crucial role in maintaining immune tolerance and preventing tissue damages by excessive immune responses. PD‐L1 is physiologically expressed and upregulated in keratinocytes (KCs) in the oral cavity. We here investigated the contribution of PD‐L1 that was overexpressed in gingival basal KCs in a ligature‐induced periodontitis model.

Methods

Wild‐type (WT) BALB/c and K14/PD‐L1 transgenic (tg) mice, in which PD‐L1 was overexpressed in basal KCs under control of the keratin 14 promoter, were used. To induce periodontitis, a 9‐0 silk ligature was placed around the upper right second molar, and lipopolysaccharide from Porphyromonas gingivalis was applied on the suture. Gingival tissues were collected on day 7, after which histological analyses were performed, including by hematoxylin and eosin and tartrate‐resistant acid phosphate staining (TRAP) and quantitative PCR for proinflammatory cytokines and bone metabolism‐related genes. Alveolar bone loss at 7 weeks after ligature placement was assessed by micro‐computed tomography analysis.

Results

PD‐L1 was overexpressed in the basal KCs of all gingival epithelia in K14/PD‐L1tg mice. Early ligature‐induced periodontal inflammation, as assessed based on histological changes, elevation of proinflammatory cytokine (IL‐1β, IL‐6, TNF‐α) expression, periodontal ligament degeneration, and osteoclastogenesis as assessed by Rankl and Opg expression and TRAP+ cells, was markedly impaired in K14/PD‐L1tg mice. Alveolar bone resorption at a late time point was also clearly minimized in K14/PD‐L1tg mice.

Conclusion

Overexpression of PD‐L1 in gingival basal keratinocytes in K14/PD‐L1tg mice reduces periodontal inflammation and alveolar bone resorption in a ligature‐induced periodontitis model.

PD-1 Expression Defines Epidermal CD8+CD103+ T Cells Preferentially Producing IL-17A and Using Skewed TCR Repertoire in Psoriasis

In psoriasis, CD8⁺CD103⁺ memory T cells residing in the epidermis represent an effector population capable of maintaining the condition and driving a recurrence of the disease. Tissue-infiltrating CD8⁺ T cells expressing PD-1 are regarded as antigen-primed effector cells in others chronic inflammatory diseases. However, the expression and significance of PD-1 on skin-infiltrating CD8⁺ T cells in human psoriasis is not known. By analyzing skin-infiltrating T cells from human psoriasis, we found that active psoriatic epidermis contained PD-1 expressing CD8⁺CD103⁺ T cells that correlated with the disease severity and histopathology. PD-1⁺CD8⁺CD103⁺ T cells possessed a canonical psoriasis-specific resident memory phenotype with IL-23R expression and produced IL-17A, whereas PD-1^-CD8⁺CD103⁺ T cells preferentially produced IFN-γ. The diversity of skin-infiltrating T cells was dominated by CD4⁺ T cells, while CD8⁺ T cells, especially CD8⁺CD103⁺T cells, represented an oligoclonal population in active psoriasis. In addition, PD-1⁺CD8⁺CD103⁺T cells used different TCR Vβs from PD-1^-CD8⁺CD103⁺T cells counterpart. In the early resolved lesion, the composition and functional status of PD-1⁺CD8⁺CD103⁺T cells were markedly altered, while PD-1^-CD8⁺CD103⁺ T cells population was minimally changed. Collectively, PD-1 expression delineates a putative pathogenic subset of epidermal CD8⁺CD103⁺ T cells, which possibly play a role in psoriasis pathogenesis.

PD-L1 on mast cells suppresses effector CD8+ T-cell activation in the skin in murine contact hypersensitivity

BACKGROUND

The programmed cell death-1 (PD-1)/programmed death ligand 1 (PD-L1) pathway is known to inhibit the activation of effector CD8⁺ T cells. However, just how this regulatory pathway is involved in the pathophysiology of CD8⁺ T-cell-mediated inflammatory skin diseases remains unclear.

OBJECTIVE

Our aim was to elucidate the mechanisms by which the PD-1/PD-L1 pathway exerts its regulatory roles in CD8⁺ T-cell-mediated cutaneous immune responses.

METHODS

PD-L1-deficient (Pdl1^-/-) mice were used for the murine contact hypersensitivity model. Inflammatory responses such as IFN-γ production from CD8⁺ T cells in the skin was evaluated by flow cytometry.

RESULTS

Compared with wild-type mice, Pdl1^-/- mice exhibited exacerbated ear swelling and increased numbers of IFN-γ⁺ CD8⁺ T cells in the skin. Adoptive T-cell transfer experiments revealed the involvement of the PD-1/PD-L1 pathway in the elicitation phase of contact hypersensitivity. Bone marrow chimera experiments showed that PD-L1 on radioresistant cells was responsible for this regulatory pathway. Flow cytometric analysis revealed that among the radioresistant cells in the skin, PD-L1 was most highly expressed on mast cells (MCs) before and after elicitation. Administration of anti-PD-L1 blocking antibody during the elicitation phase significantly enhanced ear swelling responses and increased the number of IFN-γ⁺CD8⁺ T cells in the skin of wild-type mice, whereas no significant effects were observed in MC-deficient (WBB6F1/J-Kit^W/Kit^W-v/J and C57BL/6-KitW^-sh/W^-sh) mice. The high level of expression of PD-L1 on human skin MCs was confirmed by database analysis and immunohistochemical analysis.

CONCLUSION

PD-L1 on MCs negatively regulates CD8⁺ T-cell activation in the skin.

Inactivation of the PD-1-Dependent Immunoregulation in Mice Exacerbates Contact Hypersensitivity Resembling Immune-Related Adverse Events

Blockade of PD-1, an indispensable physiological immunoregulatory mechanism, enhances immune activities and is widely used in the immunotherapy of cancer. This treatment often accompanies inflammatory complication called immune-related adverse events (irAE), most frequently in the skin. To analyze how skin inflammation develops by the blockade of PD-1-dependent immunoregulation, we studied the exacerbation of oxazolone-induced contact hypersensitivity by PD-L1 blockade. The inactivation of PD-1 signaling enhanced swelling of the skin with massive CD8⁺ T cell infiltration. Among PD-1-expressing cells, T cells were the predominant targets of anti-PD-L1 mAb treatment since PD-L1 blockade did not affect skin inflammation in RAG2^-/- mice. PD-L1 blockade during immunization with oxazolone significantly promoted the development of hapten-reactive T cells in the draining lymph nodes. The enhancement of local CD8⁺ T cell-dominant immune responses by PD-L1 blockade was correlated with the upregulation of CXCL9 and CXCL10. Challenges with a low dose of oxazolone did not demonstrate any significant dermatitis; however, the influence of PD-L1 blockade on T cell immunity was strong enough to cause the emergence of notable dermatitis in this suboptimal dosing, suggesting its relevance to dermal irAE development. In the low-dose setting, the blockade of CXCR3, receptor of CXCL9/10, prevented the induction of T cell-dominant inflammation by anti-PD-L1 mAb. This experimental approach reproduced CD8⁺ T cell-dominant form of cutaneous inflammation by the blockade of PD-L1 that has been observed in dermal irAE in human patients.

Selective Expansion of Double-Negative iNKT Cells Inhibits the Development of Atopic Dermatitis in Vα14 TCR Transgenic NC/Nga Mice by Increasing Memory-Type CD8+ T and Regulatory CD4+ T Cells

Spontaneous development of atopic dermatitis (AD) in NC/Nga (NC) mice has been attributed to a deficiency in invariant NK T (iNKT) cells. To elucidate the precise role of iNKT cells in AD development of NC mice, we employed two distinct murine models of iNKT cell over-representation: Vβ8 TCR congenic and Vα14 TCR transgenic NC mice. We found that Vα14 TCR transgenic (Vα14^Tg) but not Vβ8 TCR congenic (Vβ8^Cg) NC mice exhibited reduced AD development, which was attributed to both quantitative and qualitative changes in iNKT cells such as a biased expansion of the double-negative iNKT subset. Adoptive transfer experiments confirmed that iNKT cells from Vα14^Tg mice but not from Vβ8^Cg mice were responsible for protecting NC mice from AD development. Double-negative iNKT cells from Vα14^Tg NC mice showed a T helper type-1‒dominant cytokine profile, which may account for the expansion of CD4⁺ regulatory T cells and memory-type CD8⁺ T cells. Furthermore, the adoptive transfer of CD8⁺ T cells from Vα14^Tg NC mice into AD-susceptible wild-type NC mice suppressed AD in recipient NC mice. Taken together, our results have identified double-negative iNKT cells as promising cellular targets to prevent AD pathogenesis.

Dendritic cells treated with a prostaglandin I2 analog, iloprost, promote antigen-specific regulatory T cell differentiation in mice

Iloprost, a stable prostaglandin I₂ (PGI₂) analog, can inhibit allergic inflammation in an ovalbumin (OVA)-induced asthma model via inhibition of airway dendritic cell (DC) function. However, the underlying mechanism of PGI₂ signaling-mediated immunosuppression remains unclear. This study explored whether iloprost-treated DCs can suppress inflammation by promoting antigen-specific regulatory T cell (Treg) differentiation through PGI₂-G-protein-coupled receptor (IP). We established an allergic lung inflammation model using a hydrogel biomaterial delivery system and observed that iloprost significantly suppressed OVA-induced Th2 lung inflammation and increased the frequency of OVA-specific Tregs in vivo. We further observed that iloprost-treated DCs displayed tolerogenic characteristics, including low inflammatory cytokine (IL-12, TNF-α, IL-6, IL-23) expression levels, high anti-inflammatory cytokine (IL-10) production, and a semimature phenotype. In addition, iloprost-treated DCs increased OVA-specific CD4⁺Foxp3⁺ T cell differentiation from naïve T cells in an IP-dependent pathway in vitro and in vivo. Blocking experiments showed that iloprost-treated DCs promoted Treg differentiation, at least in part, through programmed death ligand 1 (PD-L1), whereas iloprost-induced PD-L1 expression in DCs was through the IP receptor. Furthermore, iloprost treatment suppressed DC-mediated airway inflammation and increased the frequency of OVA-specific Tregs through PD-L1 in vivo. Taken together, these results show that PGI₂-IP signaling mediated by iloprost in DCs may lead to immune tolerance, suggesting that the PGI₂ analog has the potential to be applied therapeutically for tolerogenic DC immunotherapy in autoimmune diseases or allergic asthma.

Silencing of PD-L2/B7-DC by Topical Application of Small Interfering RNA Inhibits Elicitation of Contact Hypersensitivity

PD-L2 is a ligand for the immune checkpoint receptor PD-1; however, its regulatory function is unclear. We previously reported that silencing of CD86 in cutaneous dendritic cells by topical application of small interfering RNA (siRNA) inhibits the elicitation of contact hypersensitivity (CHS). Here, we investigated the effects of topical application of PD-L2 siRNA on allergic skin disease. PD-L2 was induced in dendritic cells concurrently with the elevation of major histocompatibility complex class II and CD86 expression. Topical application of PD-L2 siRNA inhibited the elicitation of CHS by suppressing early proinflammatory cytokine expression and migration of hapten-carrying dendritic cells into lymph nodes. Local injection of neutralizing anti-PD-L2 mAb inhibited CHS to the same extent. PD-L2 siRNA treatment inhibited CHS in PD-1/PD-L1 double knockout mice and in the sensitized T-cell-transferred skin. These results suggest that the effects of PD-L2 silencing are independent of PD-1 but dependent on local memory T cells. Most of the inhibitory effects of PD-L2 and CD86 silencing on CHS were comparable, but PD-L2 siRNA treatment did not inhibit atopic disease-like manifestations and T helper type 2 responses in NC/Nga mice. Our results suggest that PD-L2 in cutaneous dendritic cells acts as a costimulator rather than a regulator. Local PD-L2 silencing by topical application of siRNA represents a therapeutic approach for contact allergy.

Mesenchymal stem cells inhibit T cell activation by releasing TGF-β1 from TGF-β1/GARP complex

Intervention with mesenchymal stem cells (MSCs) reveals a promising therapeutic tool to treat transplantation and autoimmune disease due to their immunoregulation capability. But the mechanisms of action are not fully investigated yet. Transforming growth factor-β1 (TGF-β1) exhibit multiple effects in migration, differentiation, and immunomodulation of MSCs. Glycoprotein A repetitions predominant (GARP) is an important marker of activated Treg (regulatory T cells). GARP binds latent TGF-β1 to regulate its activation, which is the indispensable step in Treg suppressing effector T cells. So far we don't know whether GARP present on MSCs and its association with MSCs function. Our study show that MSCs express GARP which binds latent TGF-β1 on their cell surface. We also found that TGF-β1+/- MSCs produce less TGF-β1 and exhibit reduced capacity in inhibiting T cells. When TGF-β1 signaling pathway was blocked, MSCs show decreased activity in inhibiting T cells. Importantly, silencing GARP expression distinctively damaged the capacity of MSCs to inhibit IFN-γ production. These findings indicated the expression of GARP on MSCs and its functionality in activating LAP, thus demonstrating GARP as a novel biomarker and new target to improve the therapeutic efficacy of MSCs.

Unique B7-H1 expression on masticatory mucosae in the oral cavity and trans-coinhibition by B7-H1-expressing keratinocytes regulating CD4+ T cell-mediated mucosal tissue inflammation

The PD-1/B7-H1 pathway regulates immune responses and maintains homeostasis. Here, we identified a unique expression of B7 homolog 1 (B7-H1) on masticatory mucosae in the oral cavity. B7-H1 was physiologically expressed on the dorsal surface of the tongue, gingiva, and hard palate. Other squamous epithelia and other structures of the epithelia did not express B7-H1 in the steady state. Physiological B7-H1 expression on masticatory mucosae was limited on prickle cells, and its expression on basal keratinocytes (KCs) was strictly regulated. B7-H1 on prickle cells was upregulated by external topical stimuli, but B7-H1 on basal KCs was induced only by internal stimuli via infiltrating cells. The blocking of KC-associated B7-H1 or the lack of programmed cell death-1 (PD-1) on tissue effector CD4⁺ T cells in mice lacking B7-H1 on immune cells drastically exacerbated the tissue inflammation induced by topical OVA painting as an exogenous antigen, indicating direct interaction with KCs and CD4⁺ T cells. Trans-coinhibitory signals by KCs may modulate local T-cell/dendritic cell activation, resulting in inhibition of T-cell responses in both peripheral and secondary lymphoid tissues. Careful control of B7-H1 induction in KCs may play a crucial role in the protection from CD4⁺ T cell-mediated tissue inflammation by exogenous antigens delivered from the mucosal surface.

Immune-Modulation by Epidermal Growth Factor Receptor Inhibitors: Implication on Anti-Tumor Immunity in Lung Cancer

Skin toxicity is the most common toxicity caused by Epidermal Growth Factor Receptor (EGFR) inhibitors, and has been associated with clinical efficacy. As EGFR inhibitors enhance the expression of antigen presenting molecules in affected skin keratinocytes, they may concurrently facilitate neo-antigen presentation in lung cancer tumor cells contributing to anti-tumor immunity. Here, we investigated the modulatory effect of the EGFR inhibitor, erlotinib on antigen presenting molecules and PD-L1, prominent immune checkpoint protein, of skin keratinocytes and lung cancer cell lines to delineate the link between EGFR signaling pathway inhibition and potential anti-tumor immunity. Erlotinib up-regulated MHC-I and MHC-II proteins on IFNγ treated keratinocytes but abrogated IFNγ-induced expression of PD-L1, suggesting the potential role of infiltrating autoreactive T cells in the damage of keratinocytes in affected skin. Interestingly, the surface expression of MHC-I, MHC-II, and PD-L1 was up-regulated in response to IFNγ more often in lung cancer cell lines sensitive to erlotinib, but only expression of PD-L1 was inhibited by erlotinib. Further, erlotinib significantly increased T cell mediated cytotoxicity on lung cancer cells. Lastly, the analysis of gene expression dataset of 186 lung cancer cell lines from Cancer Cell Line Encyclopedia demonstrated that overexpression of PD-L1 was associated with sensitivity to erlotinib and higher expression of genes related to antigen presenting pathways and IFNγ signaling pathway. Our findings suggest that the EGFR inhibitors can facilitate anti-tumor adaptive immune responses by breaking tolerance especially in EGFR driven lung cancer that are associated with overexpression of PD-L1 and genes related to antigen presentation and inflammation.

The role of lymphocytes in the development and treatment of alopecia areata

Alopecia areata (AA) development is associated with both innate and adaptive immune cell activation, migration to peri- and intra-follicular regions, and hair follicle disruption. Both CD4(+) and CD8(+) lymphocytes are abundant in AA lesions; however, CD8(+) cytotoxic T lymphocytes are more likely to enter inside hair follicles, circumstantially suggesting that they have a significant role to play in AA development. Several rodent models recapitulate important features of the human autoimmune disease and demonstrate that CD8(+) cytotoxic T lymphocytes are fundamentally required for AA induction and perpetuation. However, the initiating events, the self-antigens involved, and the molecular signaling pathways, all need further exploration. Studying CD8(+) cytotoxic T lymphocytes and their fate decisions in AA development may reveal new and improved treatment approaches.

Cutting Edge: PD-1 Regulates Imiquimod-Induced Psoriasiform Dermatitis through Inhibition of IL-17A Expression by Innate γδ-Low T Cells

Programmed cell death 1 (PD-1) is a key regulatory molecule that has been targeted in human cancers, including melanoma. In clinical testing, Abs against PD-1 have resulted in psoriasiform dermatitis (PsD). To determine whether PD-1 regulates PsD, we compared skin responses of PD-1-deficient (PD-1KO) mice and wild-type (WT) controls in an imiquimod (IMQ)-induced murine model of psoriasis. PD-1KO mice showed severe epidermal hyperplasia, greater neutrophilic infiltration, and higher expression of Th17 cytokines (versus WT mice). IMQ exposure increased PD-1 expression by skin γδ-low (GDL) T cells and enhanced expression of PD-L1 by keratinocytes. Three-fold increases in the percentage of IL-17A(+) GDL T cells were observed in skin cell suspensions derived from IMQ-treated PD-1KO mice (versus WT controls), suggesting that the lack of PD-1 has a functional effect not only on αβ T cells, but also on GDL T cells, and that PD-1 may play a regulatory role in PsD.

Programmed cell death 1 (PD-1) regulates the effector function of CD8 T cells via PD-L1 expressed on target keratinocytes

Programmed cell death 1 (PD-1) is an inhibitory molecule expressed by activated T cells. Its ligands (PD-L1 and -L2; PD-Ls) are expressed not only by a variety of leukocytes but also by stromal cells. To assess the role of PD-1 in CD8 T cell-mediated diseases, we used PD-1-knockout (KO) OVA-specific T cell-receptor transgenic (Tg) CD8 T cells (OT-I cells) in a murine model of mucocutaneous graft-versus-host disease (GVHD). We found that mice expressing OVA on epidermal keratinocytes (K14-mOVA mice) developed markedly enhanced GVHD-like disease after transfer of PD-1-KO OT-I cells as compared to those mice transferred with wild-type OT-I cells. In addition, K14-mOVA × OT-I double Tg (DTg) mice do not develop GVHD-like disease after adoptive transfer of OT-I cells, while transfer of PD-1-KO OT-I cells caused GVHD-like disease in a Fas/Fas-L independent manner. These results suggest that PD-1/PD-Ls-interactions have stronger inhibitory effects on pathogenic CD8 T cells than does Fas/Fas-L-interactions. Keratinocytes from K14-mOVA mice with GVHD-like skin lesions express PD-L1, while those from mice without the disease do not. These findings reflect the fact that primary keratinocytes express PD-L1 when stimulated by interferon-γ in vitro. When co-cultured with K14-mOVA keratinocytes for 2 days, PD-1-KO OT-I cells exhibited enhanced proliferation and activation compared to wild-type OT-I cells. In addition, knockdown of 50% PD-L1 expression on the keratinocytes with transfection of PD-L1-siRNA enhanced OT-I cell proliferation. In aggregate, our data strongly suggest that PD-L1, expressed on activated target keratinocytes presenting autoantigens, regulates autoaggressive CD8 T cells, and inhibits the development of mucocutaneous autoimmune diseases.

Cell type-specific targeting dissociates the therapeutic from the adverse effects of protein kinase inhibition in allergic skin disease

The kinase p38α, originally identified because of its endotoxin- and cytokine-inducible activity and affinity for antiinflammatory compounds, has been posited as a promising therapeutic target for various immune-mediated disorders. In clinical trials, however, p38α inhibitors produced adverse skin reactions and other toxic effects that often outweighed their benefits. Such toxicity may arise from a perturbation of physiological functions unrelated to or even protective against the disease being treated. Here, we show that the effect of interfering with p38α signaling can be therapeutic or adverse depending on the targeted cell type. Using a panel of mutant mice devoid of p38α in distinct cell types and an experimental model of allergic skin disease, we find that dendritic cell (DC)-intrinsic p38α function is crucial for both antigen-specific T-cell priming and T-cell-mediated skin inflammation, two independent processes essential for the immunopathogenesis. By contrast, p38α in other cell types serves to prevent excessive inflammation or maintain naïve T-cell pools in the peripheral lymphoid tissues. These findings highlight a dilemma in the clinical use of p38α inhibitors, yet also suggest cell-selective targeting as a potential solution for improving their therapeutic index.

Immunoregulatory molecule B7-H1 (CD274) contributes to skin carcinogenesis

B7-H1 (CD274), a member of the B7 family of coinhibitory molecules, is often induced in human tumors and its expression is closely correlated with a poor prognosis or higher malignancy grade. Tumor-associated B7-H1 is implicated in mechanisms of immune escape. Under inflammatory conditions, B7-H1 is also inducible in normal epithelial cells, but little is known about its involvement in the conversion of normal cells to tumor cells. We recently found that skin-specific expression of B7-H1 accelerates chemically induced carcinogenesis of squamous cell carcinoma (SCC), despite impaired skin inflammatory responses, in B7-H1 transgenic (B7-H1tg) mice. B7-H1tg-derived keratinocytes (KC) and SCCs exhibited a marked reduction of E-cadherin, and B7-H1tg-originated SCCs showed elevated expression of the transcription factors Slug and Twist, suggesting that B7-H1 overexpression in KCs promotes the epithelial-mesenchymal transition and accelerates carcinogenesis. This review discusses the diverse functions of B7-H1 in carcinogenesis and cancer progression, and considers future directions for developing cancer therapy targeting B7-H1.

B7-H1 and B7-DC receptors of oral squamous carcinoma cells are upregulated by Porphyromonas gingivalis

The up-regulation of the B7-H1 receptors in host cells might influence the chronicity of inflammatory disorders that frequently precede the development of human cancers. B7-H1 expression has been detected in the majority of human cancers, leading to anergy and apoptosis of activated T cells, and enabling tumor cells to overcome host response. Porphyromonas gingivalis (P. gingivalis), a putative periodontal pathogen, is an etiologic agent of periodontitis and expresses a variety of virulence factors. In this study, the expression of B7-H1 and B7-DC receptors on squamous cell carcinoma cells SCC-25 and BHY and primary human gingival keratinocytes (PHGK) was analyzed after infection with two virulent P. gingivalis strains in vitro. After 48h, the cells were stained with antibodies for human B7-H1 and B7-DC and further analyzed by flow cytometry. RNA was extracted and gene expression of B7-H1 or B7-DC was quantified by real time PCR. After infection with P. gingivalis, both B7-H1 and B7-DC receptors were up-regulated. The mean fluorescence intensity (MFI) increased from 4.5 to 9.9 (B7-H1) and from 6.9 to 15.0 (B7-DC) (p<0.05, respectively) in SCC-25 cells. PHGK showed an increase from 4.8 to 12.4 (B7-H1) and from 5.5 to 15.6 (B7-DC) (p<0.05, respectively). Streptococcus salivarius K12, a commensal bacterium, caused no up-regulation. After 24h, the expression of B7H1 and B7-DC mRNA in infected cells, normalized to GAPDH and in relation to non-infected cells, was 6.4 fold (B7-H1) and 8.6 fold (B7-DC) higher. In PHGK B7-H1/DC mRNA expression increased 8.2 fold (B7-H1) and 5.9 fold (B7DC) (p<0.05) respectively. The results of the study demonstrate that in contrast to S. salivarius K12 virulent P. gingivalis strains are able to induce the expression of the B7-H1 and B7-DC receptors in squamous carcinoma cells and human gingival keratinocytes, which might facilitate immune evasion by oral cancers.

Role of PD-L1 and PD-L2 in allergic diseases and asthma

Asthma is the result of chronic airway inflammation associated predominantly with CD4+ cells, eosinophils, mast cells, and basophils. Several T-cells subsets, including NKT cells, play a critical role in orchestrating the inflammation in the airways predominantly, by secreting interleukin-4 and interleukin-13. Recently, programmed death-1 (PD-1) with its ligands, programmed death ligand B7H1 (PD-L1) and B7DC (PD-L2), was shown to regulate T-cell activation and tolerance. PD-1 has been characterized as a negative regulator of conventional CD4+T cells. In addition, the relative roles of PD-L1 and PD-L2 in regulating the activation and function of T cells have recently been characterized. Recent studies have demonstrated that PD-L1 and PD-L2 have important but opposing roles in modulating and polarizing T-cell functions in airway hyperreactivity. Whereas the severity of asthma is greatly enhanced in absence of PD-L2, PD-L1 deficiency resulted in reduced airway hyperresponsiveness and only minimal inflammation. This observation is partially because of the polarization of NKT cells in PD-L1- and PD-L2-deficient mice. This review will discuss the recent literature regarding the role of PD-L1 and PD-L2 in allergic disease and asthma. Current understanding of the role of PD ligands in allergic asthma gives impetus to the development of novel therapeutic approaches.