Activated CTLA-4-independent immunosuppression of Treg cells disturbs CTLA-4 blockade-mediated antitumor immunity.

Combination therapy with anti-cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and anti-programmed death-1 (PD-1) monoclonal antibodies (mAbs) has dramatically improved the prognosis of patients with multiple types of cancer, including renal cell carcinoma (RCC). However, more than half of RCC patients fail to respond to this therapy. Regulatory T cells (Treg cells) are a subset of highly immunosuppressive CD4+ T cells that promote the immune escape of tumors by suppressing effector T cells in the tumor microenvironment (TME) through various mechanisms. CTLA-4 is constitutively expressed in Treg cells and is regarded as a key molecule for Treg-cell-mediated immunosuppressive functions, suppressing antigen-presenting cells by binding to CD80/CD86. Reducing Treg cells in the TME with an anti-CTLA-4 mAb with antibody-dependent cellular cytotoxicity (ADCC) activity is considered an essential mechanism to achieve tumor regression. In contrast, we demonstrated that CTLA-4 blockade without ADCC activity enhanced CD28 costimulatory signaling pathways in Treg cells and promoted Treg-cell proliferation in mouse models. CTLA-4 blockade also augmented CTLA-4-independent immunosuppressive functions, including cytokine production, leading to insufficient antitumor effects. Similar results were also observed in human peripheral blood lymphocytes and tumor-infiltrating lymphocytes from patients with RCC. Our findings highlight the importance of Treg-cell depletion to achieve tumor regression in response to CTLA-4 blockade therapies.

Combination therapy with hydrogen peroxide and irradiation promotes an abscopal effect in mouse models.

Hydrogen peroxide (H2 O2 ) induces oxidative stress and cytotoxicity, and can be used for treating cancers in combination with radiotherapy. A product comprising H2 O2 and sodium hyaluronate has been developed as a radiosensitizer. However, the effects of H2 O2 on antitumor immunity remain unclear. To investigate the effects of H2 O2 , especially the abscopal effect when combined with radiotherapy (RT), we implanted murine tumor cells simultaneously in two locations in mouse models: the hind limb and back. H2 O2 mixed with sodium hyaluronate was injected intratumorally, followed by irradiation only at the hind limb lesion. No treatment was administered to the back lesion. The H2 O2 /RT combination significantly reduced tumor growth at the noninjected/nonirradiated site in the back lesion, whereas H2 O2 or RT individually did not reduce tumor growth. Flow cytometric analyses of the tumor-draining lymph nodes in the injected/irradiated areas showed that the number of dendritic cells increased significantly with maturation in the H2 O2 /RT combination group. In addition, analyses of tumor-infiltrating lymphocytes showed that the number of CD8+ (cluster of differentiation 8) T cells and the frequency of IFN-γ+ (interferon gamma) CD8+ T cells were higher in the noninjected/nonirradiated tumors in the H2 O2 /RT group compared to those in the other groups. PD-1 (programmed death receptor 1) blockade further increased the antitumor effect against noninjected/nonirradiated tumors in the H2 O2 /RT group. Intratumoral injection of H2 O2 combined with RT therefore induces an abscopal effect by activating antitumor immunity, which can be further enhanced by PD-1 blockade. These findings promote the development of H2 O2 /RT therapy combined with cancer immunotherapies, even for advanced cancers.

A variety of 'exhausted' T cells in the tumor microenvironment.

In T-cell biology, 'exhaustion' was initially described as a hyporesponsive state in CD8+ T cells during chronic infections. Recently, exhaustion has been recognized as a T-cell dysfunctional state in the tumor microenvironment (TME). The term 'exhaustion' is used mainly to refer to effector T cells with a reduced capacity to secrete cytokines and an increased expression of inhibitory receptors. The up-regulation of exhaustion-related inhibitory receptors, including programmed cell death protein 1 (PD-1), in such T cells has been associated with the development of tumors, prompting the development of immune checkpoint inhibitors. In addition to CD8+ T cells, CD4+ T cells, including the regulatory T (Treg) cell subset, perform a wide variety of functions within the adaptive immune system. Up-regulation of the same inhibitory receptors that are associated with CD8+ T-cell exhaustion has also been identified in CD4+ T cells in chronic infections and cancers, suggesting a similar CD4+ T-cell exhaustion phenotype. For instance, high expression of PD-1 has been observed in Treg cells in the TME, and such Treg cells can play an important role in the resistance to PD-1 blockade therapies. Furthermore, recent progress in single-cell RNA sequencing has shown that CD4+ T cells with cytotoxic activity are also vulnerable to exhaustion. In this review, we will discuss novel insights into various exhausted T-cell subsets, which could reveal novel therapeutic targets and strategies to induce a robust anti-tumor immune response.

Mechanisms of resistance to immune checkpoint inhibitors.

Immune checkpoint inhibitors (ICIs) are effective for various types of cancer, and their application has led to paradigm shifts in cancer treatment. While many patients can obtain clinical benefits from ICI treatment, a large number of patients are primarily resistant to such treatment or acquire resistance after an initial response. Thus, elucidating the resistance mechanisms is warranted to improve the clinical outcomes of ICI treatment. ICIs exert their antitumor effects by activating T cells in the tumor microenvironment. There are various resistance mechanisms, such as insufficient antigen recognition by T cells, impaired T-cell migration and/or infiltration, and reduced T-cell cytotoxicity, most of which are related to the T-cell activation process. Thus, we classify them into three main mechanisms: resistance mechanisms related to antigen recognition, T-cell migration and/or infiltration, and effector functions of T cells. In this review, we summarize these mechanisms of resistance to ICIs related to the T-cell activation process and progress in the development of novel therapies that can overcome resistance.

Immunomodulatory and direct activities of ropeginterferon alfa-2b on cancer cells in mouse models of leukemia.

Although ropeginterferon alfa-2b has recently been clinically applied to myeloproliferative neoplasms with promising results, its antitumor mechanism has not been thoroughly investigated. Using a leukemia model developed in immunocompetent mice, we evaluated the direct cytotoxic effects and indirect effects induced by ropeginterferon alfa-2b in tumor cells. Ropeginterferon alfa-2b therapy significantly prolonged the survival of mice bearing leukemia cells and led to long-term remission in some mice. Alternatively, conventional interferon-alpha treatment slightly extended the survival and all mice died. When ropeginterferon alfa-2b was administered to interferon-alpha receptor 1-knockout mice after the development of leukemia to verify the direct effect on the tumor, the survival of these mice was slightly prolonged; nevertheless, all of them died. In vivo CD4+ or CD8+ T-cell depletion resulted in a significant loss of therapeutic efficacy in mice. These results indicate that the host adoptive immunostimulatory effect of ropeginterferon alfa-2b is the dominant mechanism through which tumor cells are suppressed. Moreover, mice in long-term remission did not develop leukemia, even after tumor rechallenge. Rejection of rechallenge tumors was canceled only when both CD4+ and CD8+ T cells were removed in vivo, which indicates that each T-cell group functions independently in immunological memory. We show that ropeginterferon alfa-2b induces excellent antitumor immunomodulation in hosts. Our finding serves in devising therapeutic strategies with ropeginterferon alfa-2b.

Wilms Tumor 1 (WT1) mRNA Expression Level at Diagnosis Is a Significant Prognostic Marker in Elderly Patients with Myelodysplastic Syndrome.

BACKGROUND/AIMS: A high expression of Wilms tumor 1 (WT1) mRNA occurs in most cases of acute leukemia and myelodysplastic syndrome (MDS). Although there are many reports suggesting that acute myeloid leukemia patients with high expression levels of WT1 mRNA have a relatively poor long-term survival, there are few reports addressing the relationship between WT1 levels and prognosis in MDS. METHODS: We retrospectively analyzed 42 elderly patients with MDS whose WT1 levels at diagnosis were available, and we assessed the relationships between WT1 levels in peripheral blood and preexisting prognostic factors such as World Health Organization prognostic scores and Revised International Prognostic Scoring System risk categories, bone marrow blast percentages, and chromosomal abnormalities linked to a poor prognosis. We also evaluated the relationship between WT1 levels and prognosis. RESULTS: WT1 levels were significantly different between high- and low-risk MDS patients (p < 0.05). There was a trend towards a significant difference between those with and those without poor prognostic chromosomal rearrangements (p = 0.051). Moreover, the overall survival and progression-free survival were significantly worse in elderly patients with higher levels of WT1 (p = 0.00039 and p = 0.00077, respectively). CONCLUSIONS: The WT1 mRNA expression level at diagnosis may be a significant independent prognostic marker for elderly patients with MDS.

Reversible dasatinib-related pulmonary arterial hypertension in a CML patient.

A 59-year-old man diagnosed with the chronic phase of chronic myeloid leukemia (CML) in June 2011 was started on dasatinib (100 mg/day). He had no signs of pleural effusion (PE) or right heart failure before treatment, but symptoms of PE and dyspnea (New York Heart Association class III) appeared in January 2013 and May 2014, respectively. Doppler transthoracic echocardiography and right heart catheterization revealed pulmonary arterial hypertension (PAH) with an estimated pulmonary artery systolic pressure (PASP) of 80 mmHg and estimated mean pulmonary artery pressure of 29 mmHg. Rheumatoid factor, antinuclear antibody, dsDNA antibody, and SCL70 were not elevated, and computed tomography confirmed the absence of a pulmonary embolism. Therefore, dasatinib-related PAH was diagnosed and treatment with this agent was discontinued. The PASP had decreased to 51 and 40 mmHg at one month and one year, respectively, after dasatinib discontinuation. This patient developed PAH while receiving dasatinib administration and only discontinuation of this agent improved his symptoms. The possibility that dasatinib can cause PAH must be considered before administering this agent to patients with CML.

Stem-like progenitor and terminally differentiated TFH-like CD4+ T cell exhaustion in the tumor microenvironment.

Immune checkpoint inhibitors exert clinical efficacy against various types of cancer through reinvigoration of exhausted CD8+ T cells that attack cancer cells directly in the tumor microenvironment (TME). Using single-cell sequencing and mouse models, we show that CXCL13, highly expressed in tumor-infiltrating exhausted CD8+ T cells, induces CD4+ follicular helper T (TFH) cell infiltration, contributing to anti-tumor immunity. Furthermore, a part of the TFH cells in the TME exhibits cytotoxicity and directly attacks major histocompatibility complex-II-expressing tumors. TFH-like cytotoxic CD4+ T cells have high LAG-3/BLIMP1 and low TCF1 expression without self-renewal ability, whereas non-cytotoxic TFH cells express low LAG-3/BLIMP1 and high TCF1 with self-renewal ability, closely resembling the relationship between terminally differentiated and stem-like progenitor exhaustion in CD8+ T cells, respectively. Our findings provide deep insights into TFH-like CD4+ T cell exhaustion with helper progenitor and cytotoxic differentiated functions, mediating anti-tumor immunity orchestrally with CD8+ T cells.

CD106 in tumor-specific exhausted CD8+ T cells mediates immunosuppression by inhibiting TCR signaling.

T cell exhaustion is a major contributor to immunosuppression in the tumor microenvironment (TME). Blockade of key regulators of T cell exhaustion, such as PD-1, can reinvigorate tumor-specific T cells and activate anti-tumor immunity in various types of cancer. Here, we identified that CD106 was specifically expressed in exhausted CD8+ T cells in the TME using single-cell RNA-sequencing. High CD106 expression in the TME in clinical samples corresponded to improved response to cancer immunotherapy. CD106 in tumor-specific T cells suppressed anti-tumor immunity both in vitro and in vivo, and loss of CD106 in CD8+ T cells suppressed tumor growth and improved response to PD-1 blockade. Mechanistically, CD106 inhibited T-cell receptor (TCR) signaling by interacting with the TCR/CD3 complex and reducing its surface expression. Together, these findings provide insights into the immunosuppressive role of CD106 expressed in tumor-specific exhausted CD8+ T cells, identifying it as a potential biomarker and therapeutic target for cancer immunotherapy.

High Expression of MHC Class I Overcomes Cancer Immunotherapy Resistance Due to IFNγ Signaling Pathway Defects.

IFNγ signaling pathway defects are well-known mechanisms of resistance to immune checkpoint inhibitors. However, conflicting data have been reported, and the detailed mechanisms remain unclear. In this study, we have demonstrated that resistance to immune checkpoint inhibitors owing to IFNγ signaling pathway defects may be primarily caused by reduced MHC-I expression rather than by the loss of inhibitory effects on cellular proliferation or decreased chemokine production. In particular, we found that chemokines that recruit effector T cells were mainly produced by immune cells rather than cancer cells in the tumor microenvironment of a mouse model, with defects in IFNγ signaling pathways. Furthermore, we found a response to immune checkpoint inhibitors in a patient with JAK-negative head and neck squamous cell carcinoma whose HLA-I expression level was maintained. In addition, CRISPR screening to identify molecules associated with elevated MHC-I expression independent of IFNγ signaling pathways demonstrated that guanine nucleotide-binding protein subunit gamma 4 (GNG4) maintained MHC-I expression via the NF-κB signaling pathway. Our results indicate that patients with IFNγ signaling pathway defects are not always resistant to immune checkpoint inhibitors and highlight the importance of MHC-I expression among the pathways and the possibility of NF-κB-targeted therapies to overcome such resistance. See related Spotlight by Haugh and Daud, p. 864.

Immunologic Significance of CD80/CD86 or Major Histocompatibility Complex-II Expression in Thymic Epithelial Tumors.

Introduction: Unresectable or recurrent thymic epithelial tumors (TETs) have a poor prognosis, and treatment options are limited. This study aimed to investigate the immunologic significance of CD80/CD86 or major histocompatibility complex class II (MHC-II) expression in TETs, as potential predictive biomarkers for immune checkpoint inhibitors (ICIs). Methods: We analyzed CD80, CD86, MHC class I (MHC-I), and MHC-II expression in TETs using immunohistochemistry and investigated their association with T-cell infiltration or ICI efficacy. In addition, we generated CD80- or MHC-II-expressing mouse tumors, evaluated the effects of ICIs, and analyzed tumor-infiltrating lymphocytes. We also performed tumor-rechallenge experiments in vivo. Results: We found that approximately 50% and 30% of TETs had high expression of CD80/CD86 and MHC-II in tumor cells, respectively, and that this expression was related to T-cell infiltration in clinical samples. In mouse models, both CD80 and MHC-II increase the effects of ICIs. In addition, senescent T cells and long-lived memory precursor effector T cells were significantly decreased and increased, respectively, in tumor-infiltrating lymphocytes from CD80-expressing tumors, and rechallenged tumors were completely rejected after the initial eradication of CD80-expressing tumors by programmed cell death protein 1 blockade. Indeed, patients with CD80-high thymic carcinoma had longer progression-free survival with anti-programmed cell death protein 1 monoclonal antibody. Conclusions: Half of the TETs had high expression of CD80/CD86 or MHC-II with high T-cell infiltration. These molecules could potentially increase the effects of ICIs, particularly inducing a durable response. CD80/CD86 and MHC-II can be predictive biomarkers of ICIs in TETs, promoting the development of drugs for such TETs.

PD-1 blockade therapy promotes infiltration of tumor-attacking exhausted T cell clonotypes.

PD-1 blockade exerts clinical efficacy against various types of cancer by reinvigorating T cells that directly attack tumor cells (tumor-specific T cells) in the tumor microenvironment (TME), and tumor-infiltrating lymphocytes (TILs) also comprise nonspecific bystander T cells. Here, using single-cell sequencing, we show that TILs include skewed T cell clonotypes, which are characterized by exhaustion (Tex) or nonexhaustion signatures (Tnon-ex). Among skewed clonotypes, those in the Tex, but not those in the Tnon-ex, cluster respond to autologous tumor cell lines. After PD-1 blockade, non-preexisting tumor-specific clonotypes in the Tex cluster appear in the TME. Tumor-draining lymph nodes (TDLNs) without metastasis harbor a considerable number of such clonotypes, whereas these clonotypes are rarely detected in peripheral blood. We propose that tumor-infiltrating skewed T cell clonotypes with an exhausted phenotype directly attack tumor cells and that PD-1 blockade can promote infiltration of such Tex clonotypes, mainly from TDLNs.

Mixed Response to Cancer Immunotherapy is Driven by Intratumor Heterogeneity and Differential Interlesion Immune Infiltration.

Some patients experience mixed response to immunotherapy, whose biological mechanisms and clinical impact have been obscure. We obtained two tumor samples from lymph node (LN) metastatic lesions in a same patient. Whole exome sequencing for the both tumors and single-cell sequencing for the both tumor-infiltrating lymphocytes (TIL) demonstrated a significant difference in tumor clonality and TILs' characteristics, especially exhausted T-cell clonotypes, although a close relationship between the tumor cell and T-cell clones were observed as a response of an overlapped exhausted T-cell clone to an overlapped neoantigen. To mimic the clinical setting, we generated a mouse model of several clones from a same tumor cell line. Similarly, differential tumor clones harbored distinct TILs, and one responded to programmed cell death protein 1 (PD-1) blockade but the other did not in this model. We further conducted cohort study (n = 503) treated with PD-1 blockade monotherapies to investigate the outcome of mixed response. Patients with mixed responses to PD-1 blockade had a poor prognosis in our cohort. Particularly, there were significant differences in both tumor and T-cell clones between the primary and LN lesions in a patient who experienced tumor response to anti-PD-1 mAb followed by disease progression in only LN metastasis. Our results underscore that intertumoral heterogeneity alters characteristics of TILs even in the same patient, leading to mixed response to immunotherapy and significant difference in the outcome. Significance: Several patients experience mixed responses to immunotherapies, but the biological mechanisms and clinical significance remain unclear. Our results from clinical and mouse studies underscore that intertumoral heterogeneity alters characteristics of TILs even in the same patient, leading to mixed response to immunotherapy and significant difference in the outcome.

TIGIT/CD155 axis mediates resistance to immunotherapy in patients with melanoma with the inflamed tumor microenvironment.

BACKGROUND: Patients with cancer benefit from treatment with immune checkpoint inhibitors (ICIs), and those with an inflamed tumor microenvironment (TME) and/or high tumor mutation burden (TMB), particularly, tend to respond to ICIs; however, some patients fail, whereas others acquire resistance after initial response despite the inflamed TME and/or high TMB. We assessed the detailed biological mechanisms of resistance to ICIs such as programmed death 1 and/or cytotoxic T-lymphocyte-associated protein 4 blockade therapies using clinical samples. METHODS: We established four pairs of autologous tumor cell lines and tumor-infiltrating lymphocytes (TILs) from patients with melanoma treated with ICIs. These tumor cell lines and TILs were subjected to comprehensive analyses and in vitro functional assays. We assessed tumor volume and TILs in vivo mouse models to validate identified mechanism. Furthermore, we analyzed additional clinical samples from another large melanoma cohort. RESULTS: Two patients were super-responders, and the others acquired resistance: the first patient had a non-inflamed TME and acquired resistance due to the loss of the beta-2 microglobulin gene, and the other acquired resistance despite having inflamed TME and extremely high TMB which are reportedly predictive biomarkers. Tumor cell line and paired TIL analyses showed high CD155, TIGIT ligand, and TIGIT expression in the tumor cell line and tumor-infiltrating T cells, respectively. TIGIT blockade or CD155-deletion activated T cells in a functional assay using an autologous cell line and paired TILs from this patient. CD155 expression increased in surviving tumor cells after coculturing with TILs from a responder, which suppressed TIGIT+ T-cell activation. Consistently, TIGIT blockade or CD155-deletion could aid in overcoming resistance to ICIs in vivo mouse models. In clinical samples, CD155 was related to resistance to ICIs in patients with melanoma with an inflamed TME, including both primary and acquired resistance. CONCLUSIONS: The TIGIT/CD155 axis mediates resistance to ICIs in patients with melanoma with an inflamed TME, promoting the development of TIGIT blockade therapies in such patients with cancer.

Single-Cell Analysis of the Multicellular Ecosystem in Viral Carcinogenesis by HTLV-1.

Premalignant clonal expansion of human T-cell leukemia virus type-1 (HTLV-1)-infected cells occurs before viral carcinogenesis. Here we characterize premalignant cells and the multicellular ecosystem in HTLV-1 infection with and without adult T-cell leukemia/lymphoma (ATL) by genome sequencing and single-cell simultaneous transcriptome and T/B-cell receptor sequencing with surface protein analysis. We distinguish malignant phenotypes caused by HTLV-1 infection and leukemogenesis and dissect clonal evolution of malignant cells with different clinical behavior. Within HTLV-1-infected cells, a regulatory T-cell phenotype associates with premalignant clonal expansion. We also delineate differences between virus- and tumor-related changes in the nonmalignant hematopoietic pool, including tumor-specific myeloid propagation. In a newly generated conditional knockout mouse model recapitulating T-cell-restricted CD274 (encoding PD-L1) gene lesions found in ATL, we demonstrate that PD-L1 overexpressed by T cells is transferred to surrounding cells, leading to their PD-L1 upregulation. Our findings provide insights into clonal evolution and immune landscape of multistep virus carcinogenesis. SIGNIFICANCE: Our multimodal single-cell analyses comprehensively dissect the cellular and molecular alterations of the peripheral blood in HTLV-1 infection, with and without progression to leukemia. This study not only sheds light on premalignant clonal expansion in viral carcinogenesis, but also helps to devise novel diagnostic and therapeutic strategies for HTLV-1-related disorders.

The critical role of CD4+ T cells in PD-1 blockade against MHC-II-expressing tumors such as classic Hodgkin lymphoma.

Classic Hodgkin lymphoma (cHL) responds markedly to PD-1 blockade therapy, and the clinical responses are reportedly dependent on expression of major histocompatibility complex class II (MHC-II). This dependence is different from other solid tumors, in which the MHC class I (MHC-I)/CD8+ T-cell axis plays a critical role. In this study, we investigated the role of the MHC-II/CD4+ T-cell axis in the antitumor effect of PD-1 blockade on cHL. In cHL, MHC-I expression was frequently lost, but MHC-II expression was maintained. CD4+ T cells highly infiltrated the tumor microenvironment of MHC-II-expressing cHL, regardless of MHC-I expression status. Consequently, CD4+ T-cell, but not CD8+ T-cell, infiltration was a good prognostic factor in cHL, and PD-1 blockade showed antitumor efficacy against MHC-II-expressing cHL associated with CD4+ T-cell infiltration. Murine lymphoma and solid tumor models revealed the critical role of antitumor effects mediated by CD4+ T cells: an anti-PD-1 monoclonal antibody exerted antitumor effects on MHC-I-MHC-II+ tumors but not on MHC-I-MHC-II- tumors, in a cytotoxic CD4+ T-cell-dependent manner. Furthermore, LAG-3, which reportedly binds to MHC-II, was highly expressed by tumor-infiltrating CD4+ T cells in MHC-II-expressing tumors. Therefore, the combination of LAG-3 blockade with PD-1 blockade showed a far stronger antitumor immunity compared with either treatment alone. We propose that PD-1 blockade therapies have antitumor effects on MHC-II-expressing tumors such as cHL that are mediated by cytotoxic CD4+ T cells and that LAG-3 could be a candidate for combination therapy with PD-1 blockade.

The First Case of Non-leukemic Sarcoma Composed of Mixed-phenotype Acute Leukemia, B/myeloid, Not Otherwise Specified.

Isolated sarcoma with features of mixed-phenotype acute leukemia (MPAL) is an extremely rare disease and it can be easily misdiagnosed as lymphoma or other malignancies. We herein report the case of a 61-year-old woman with non-leukemic sarcoma of the right pleura, pretracheal lymph node, and supraclavicular lymph node with features of MPAL, B/myeloid, not otherwise specified, which was first misdiagnosed as diffuse large B cell lymphoma. After performing a detailed re-examination of the biopsy specimens, few scattered eosinophilic myelocytes allowed us to reach a correct diagnosis of MPAL and the patient was thereafter successfully treated by intensified chemotherapy followed by cord blood transplantation.

Pulmonary Involvement of Acute Myeloid Leukemia Mimicking Transfusion-related Acute Lung Injury.

Transfusion-related acute lung injury (TRALI) is defined as a new episode of acute lung injury (ALI) occurring during transfusion or within 6 hours of transfusion completion. A 66-year-old man suffering from acute myeloid leukemia developed acute respiratory distress syndrome after platelet transfusion. TRALI was diagnosed clinically, but an autopsy showed leukemic cells in diffuse pulmonary edema. Anti-human neutrophil antigen (HNA)-3a antibodies were detected in the donor serum, and the HNA-3 genotype of the patient was identified as a/a. This case was considered to represent pulmonary involvement of acute myeloid leukemia, rather than TRALI. A revision of the definition of TRALI accounting for hematological malignancies should therefore be considered.

Postinfluenza Vaccination Idiopathic Thrombocytopenic Purpura in Three Elderly Patients.

The etiologies of secondary idiopathic thrombocytopenic purpura (ITP) include infection, autoimmune disease, and immunodeficiency. We report the cases of three elderly patients who developed ITP after receiving influenza vaccinations. The platelet count of an 81-year-old woman fell to 27,000/µL after she received an influenza vaccination. A 75-year-old woman developed thrombocytopenia (5,000 platelets/µL) after receiving an influenza vaccination. An 87-year-old woman whose laboratory test values included a platelet count of 2,000/µL experienced genital bleeding after receiving an influenza vaccination. After Helicobacter pylori (HP) eradication or corticosteroid treatment, all of the patients' platelet counts increased. Influenza vaccination is an underlying etiology of ITP in elderly patients. HP eradication or corticosteroid treatment is effective for these patients. Clinicians should be aware of the association between ITP and influenza vaccinations.