Treatment of colon and lung cancer patients with ex vivo heat shock protein 70-peptide-activated, autologous natural killer cells: a clinical phase i trial

PURPOSE

The 14 amino acid sequence (aa(450-463)) TKDNNLLGRFELSG (TKD) of heat shock protein 70 (Hsp70) was identified as a tumor-selective recognition structure for natural killer (NK) cells. Incubation of peripheral blood lymphocyte cells with TKD plus low-dose interleukin 2 (IL-2) enhances the cytolytic activity of NK cells against Hsp70 membrane-positive tumors, in vitro and in vivo. These data encouraged us to test tolerability, feasibility, and safety of TKD-activated NK cells in a clinical Phase I trial.

EXPERIMENTAL DESIGN

Patients with metastatic colorectal cancer (n = 11) and non-small cell lung cancer (n = 1) who had failed standard therapies were enrolled. After ex vivo stimulation of autologous peripheral blood lymphocytes with Hsp70-peptide TKD (2 microg/ml) plus low-dose IL-2 (100 units/ml), TKD was removed by extensive washing, and activated cells were reinfused i.v. The procedure was repeated for up to six cycles, applying a dose escalation schedule in 4 patients.

RESULTS

The percentage of activated NK cells in the reinfused leukapheresis products ranged between 8 and 20% of total lymphocytes, corresponding to total NK cell counts of 0.1 up to 1.5 x 10(9). Apart from restless feeling in 1 patient and itching in 2 patients, no negative side effects were observed. Concomitant with an enhanced CD94 cell surface density, the cytolytic activity of NK cells against Hsp70 membrane-positive colon carcinoma cells was enhanced after TKD/IL-2 stimulation in 10 of 12 patients. Concerning tumor response, 1 patient was in stable disease during therapy by formal staging criteria and another patient showed stable disease in one metastases and progression in another.

CONCLUSIONS

Reinfusion of Hsp70-activated autologous NK cells is safe. Immunological results warrant additional studies in patients with lower tumor burden.

Isolation and characterization of human antigen-specific TCR alpha beta+ CD4(-)CD8- double-negative regulatory T cells

Down-regulation of immune responses by regulatory T (Treg) cells is an important mechanism involved in the induction of tolerance to allo-antigens (Ags). Recently, a novel subset of Ag-specific T-cell receptor (TCR)alpha beta+ CD4(-)CD8- (double-negative [DN]) Treg cells has been found to be able to prevent the rejection of skin and heart allografts by specifically inhibiting the function of antigraft-specific CD8+ T cells. Here we demonstrate that peripheral DN Treg cells are present in humans, where they constitute about 1% of total CD3+ T cells, and consist of both naive and Ag-experienced cells. Similar to murine DN Treg cells, human DN Treg cells are able to acquire peptide-HLA-A2 complexes from antigen-presenting cells by cell contact-dependent mechanisms. Furthermore, such acquired peptide-HLA complexes appear to be functionally active, in that CD8+ T cells specific for the HLA-A2-restricted self-peptide, Melan-A, became sensitive to apoptosis by neighboring DN T cells after acquisition of Melan-A-HLA-A2 complexes and revealed a reduced proliferative response. These results demonstrate for the first time that a sizable population of peripheral DN Treg cells, which are able to suppress Ag-specific T cells, exists in humans. DN Treg cells may serve to limit clonal expansion of allo-Ag-specific T cells after transplantation.

Impact of Thrombophilic Gene Mutations and Graft-versus-Host Disease on Thromboembolic Complications after Allogeneic Hematopoietic Stem-Cell Transplantation

BACKGROUND

Hemostatic events in patients undergoing allogeneic hematopoietic stem-cell transplantation (HSCT) increase the morbidity and mortality in this cohort. Little is known about the impact of graft-versus-host disease (GvHD) or of thrombophilic gene mutations/polymorphisms on these complications.

STUDY DESIGN

Eighty-nine allogeneic stem-cell recipients and their donors were evaluated prospectively for the presence of the factor V G1691A mutation, the prothrombin G20210A mutation, the 5,10-methylenetetrahydrofolate-reductase (MTHFR) C677T mutation, the glycoprotein IIIa PI(a1/a2) polymorphism, the fibrinogen-beta-chain 455G/A polymorphism, the plasminogen activator inhibitor-1 -675 4G/5G polymorphism, and the angiotensin-converting enzyme intron 16 I/D polymorphism. These mutations/polymorphisms and GvHD parameters were correlated to hemostatic and toxic complications after transplantation. The data were compared with those of 128 healthy controls.

RESULTS

The PAI-1 4G/4G polymorphism increases the risk for catheter thrombosis after HSCT 5.7-fold (32.2% vs. 71.4%, P<0.05). In patients with hepatic veno-occlusive disease, the frequency of the PAI-1 4G allele is also increased (83.3% vs. 55.1%, NS). Thrombophilic mutations/polymorphisms in donors do not influence complications in the corresponding recipients. The MTHFR TT genotype does not modify severity and duration of mucositis and aplasia in patients receiving methotrexate prophylaxis. Patients with chronic GvHD have a higher risk of thromboembolism (12.9% vs. 1.7%, P<0.05).

CONCLUSION

Thrombophilic gene mutations have only a moderate influence on hemostatic complications in patients undergoing HSCT. This may be because of the overwhelming immunologic impact of GvHD on hemostasis in the allogeneic transplantation setting.

High Frequency of Functionally Active Melan-A–Specific T Cells in a Patient with Progressive Immunoproteasome-Deficient Melanoma

Tumor-reactive T cells play an important role in cancer immunosurveillance. Applying the multimer technology, we report here an unexpected high frequency of Melan-A–specific CTLs in a melanoma patient with progressive lymph node metastases, consisting of 18 and 12.8% of total peripheral blood and tumor-infiltrating CD8+ T cells, respectively. Melan-A–specific CTLs revealed a high cytolytic activity against allogeneic Melan-A–expressing target cells but failed to kill the autologous tumor cells. Loading of the tumor cells with Melan-A peptide reversed the resistance to killing, suggesting impaired function of the MHC class I antigen processing and presentation pathway. Mutations of the coding region of the HLA-A2 binding Melan-A26–35 peptide or down-regulation of the MHC class I heavy chain, the antigenic peptide TAP, and tapasin could be excluded. However, PCR and immunohistochemical analysis revealed a deficiency of the immunoproteasomes low molecular weight protein 2 and low molecular weight protein 7 in the primary tumor cells, which affects the quantity and quality of generated T-cell epitopes and might explain the resistance to killing. This is supported by our data, demonstrating that the resistance to killing can be partially reversed by pre-exposure of the tumor cells to IFN-γ, which is known to induce the immunoproteasomes. Overall, this is the first report of an extremely high frequency of tumor-specific CTLs that exhibit competent T-cell–effector functions but fail to lyse the autologous tumor cells. Immunotherapeutic approaches should not only focus on the induction of a robust antitumor immune response, but should also have to target tumor immune escape mechanisms.

Induction of heme oxygenase-1 before conditioning results in improved survival and reduced graft-versus-host disease after experimental allogeneic bone marrow transplantation

Acute graft-versus-host disease (aGVHD) remains one of the main obstacles after allogeneic bone marrow transplantation (BMT). Using a well-established mouse BMT model in which aGVHD is induced across a haploidentical mismatch, we show that the expression of heme oxygenase-1 (HO-1) can be induced by cobalt-protoporphyrin IX (CoPP) in aGVHD target organs such as liver and bowel and that the induction of HO-1 before BMT results in improved overall survival and reduced aGVHD. Serum levels of proinflammatory cytokines were markedly reduced in CoPP-treated animals. Recipients displayed less damage to the intestinal mucosa, and this resulted in reduced serum lipopolysaccharide levels at day 6 after transplantation. Peritoneal cells and CD45(+) liver cells isolated from mice that received transplants strongly expressed HO-1 and displayed a reduction in the expression of activation markers such as CD11b, CD80, and major histocompatibility complex class I. This resulted in reduced T-cell activation ex vivo. These results demonstrate that the induction of HO-1 before high-dose conditioning protects the host in multiple ways and effectively ameliorates aGVHD.

Large-scale in vitro expansion of polyclonal human CD4(+)CD25high regulatory T cells

CD4(+)CD25+ regulatory T (Treg) cells are pivotal for the maintenance of self-tolerance, and their adoptive transfer gives protection from autoimmune diseases and pathogenic alloresponses after solid organ or bone marrow transplantation in murine model systems. In vitro, human CD4(+)CD25+ Treg cells display phenotypic and functional characteristics similar to those of murine CD4(+)CD25+ Treg cells: namely, hyporesponsiveness to T-cell receptor (TCR) stimulation and suppression of CD25- T cells. Thus far, the detailed characterization and potential clinical application of human CD4(+)CD25+ Treg cells have been hampered by their paucity in peripheral blood and the lack of appropriate expansion protocols. Here we describe the up to 40 000-fold expansion of highly purified human CD4(+)CD25high T cells in vitro through the use of artificial antigen-presenting cells for repeated stimulation via CD3 and CD28 in the presence of high-dose interleukin 2 (IL-2). Expanded CD4(+)CD25high T cells were polyclonal, maintained their phenotype, exceeded the suppressive activity of freshly isolated CD4(+)CD25high T cells, and maintained expression of the lymph node homing receptors L-selectin (CD62L) and CCR7. The ability to rapidly expand human CD4(+)CD25high Treg cells on a large scale will not only facilitate their further exploration but also accelerate their potential clinical application in T cell-mediated diseases and transplantation medicine.

Both donor and recipient NOD2/CARD15 mutations associate with transplant-related mortality and GvHD following allogeneic stem cell transplantation

Single nucleotide polymorphisms (SNPs) of the NOD2/CARD15 gene resulting in a diminished nuclear factor-kappaB (NF-kappaB) response to bacterial cell wall products have been associated with an increased incidence of Crohn disease. To assess a possible contribution of NOD2/CARD15 mutations to graft-versus-host disease (GvHD) and complications following allogeneic stem cell transplantation, we retrospectively typed DNA from donor/recipient pairs in 169 consecutive patients receiving transplants from related or unrelated donors. Mutated alleles were observed in 21% of patients and in 14% of donors. Cumulative incidence of 1-year, transplant-related mortality rose from 20% in donor/recipient pairs without mutated SNPs to 49% in pairs with recipient mutations (P =.03) and 59% in pairs with donor mutations (P <.005), and was highest in 12 pairs with mutated alleles in both donor and recipients (83%; P <.001). Similar associations were observed for severe overall and severe gastrointestinal GvHD. The impact of NOD2/CARD15 mutations was more prominent for HLA-identical sibling transplantations but was also observed in unrelated donor transplantation. Mutations proved to be independent risk factors for transplant-related mortality. Our findings indicate a major role of monocyte/macrophage dysfunction in the pathophysiology of GvHD and strongly suggest a future risk assessment or even donor selection through NOD2/CARD15 typing.

Probiotic effects on experimental graft-versus-host disease: let them eat yogurt

Acute graft-versus-host disease (aGVHD) often limits feasibility and outcome of allogeneic bone marrow transplantation. Current pathophysiologic concepts of aGVHD involve conditioning regimens, donor-derived T cells, proinflammatory cytokines, and bacterial lipopolysaccharide (LPS) as a major trigger for aGVHD. LPS derives mostly from gram-negative bacteria and can enter circulation through the impaired mucosal barrier after the conditioning regimen. Probiotic microorganisms have been shown to alter the composition of the intestinal microflora and thereby mediate anti-inflammatory effects. We hypothesized that modifying the enteric flora using the probiotic microorganism Lactobacillus rhamnosus GG, would ameliorate aGVHD. Here we show that oral administration of Lactobacillus rhamnosus GG before and after transplantation results in improved survival and reduced aGVHD. Furthermore, subculturing of mesenteric lymph node tissue revealed a reduced translocation of enteric bacteria. Our findings suggest that alteration of the intestinal microflora plays an important role in the initiation of experimental aGVHD.

Microsatellite Instability Predicts Poor Short-Term Survival in Patients with Advanced Breast Cancer after High-Dose Chemotherapy and Autologous Stem-Cell Transplantation

PURPOSE

The purpose is to define molecular prognostic factors in patients with advanced breast cancer treated with high-dose chemotherapy (HDCT) and autologous stem cell transplantation (ASCT).

EXPERIMENTAL DESIGN

Thirty-nine patients with breast cancer and extensive lymph node (level III) and/or systemic metastases from a prospective single-center study of sequential HDCT/ASCT were studied. Microsatellite analysis was performed after laser microdissection using 15 markers selected for sensitive detection of microsatellite instability (MSI) in breast cancer. Exons 5-9 of the P53 gene were directly sequenced. Expression of P53, HER-2/neu, and the mismatch repair proteins hMSH2 and hMLH1 was evaluated by immunohistochemistry.

RESULTS

MSI of at least three markers was detected in 13 of 39 patients (33%) and was predominantly found at tetranucleotide markers. All MSI-positive tumors showed normal expression of hMSH2 and hMLH1. Complete sequence analysis of exons 5-9 of the P53 gene was successful in 34 cases; 18% (n = 6) revealed a mutation. Overexpression of HER-2/neu and P53 was observed in 7 (22%) and 12 (46%) of 26 evaluated cases, respectively. The presence of MSI strongly correlated with shorter overall survival (OS; P = 0.0004) and progression-free survival (PFS; P = 0.02). None of the other investigated clinical or molecular factors correlated with OS in univariate analyses, with the exception of menopausal status and previous adjuvant chemotherapy. Testing various multivariate Cox regression models, MSI remained a highly significant, independent, and adverse risk factor for OS.

CONCLUSIONS

MSI is frequent in advanced breast cancer and could be an indicator of chemotherapy resistance and poor prognosis in breast cancer patients treated with HDCT/ASCT.

Inverse regulation of the ADAM-family members, decysin and MADDAM/ADAM19 during monocyte differentiation

Two members of the ADAM (a disintegrin and metalloprotease)-family, MADDAM and decysin, were described as dendritic cell (DC) maturation markers. We are interested in monocyte differentiation and investigated in particular the expression pattern of both genes during the differentiation of human monocytes into DC and macrophages (MAC). Both genes are weakly expressed in freshly isolated monocytes. In immature DC decysin mRNA was absent, even after induction of the terminal differentiation of DC by CD40L or tumour necrosis factor-alpha (TNF-alpha). Only in DC maturated by lipopolysaccharide (LPS) strong signals of decysin mRNA were detected. However, MADDAM mRNA was expressed in immature DC and the expression was markedly increased after induction of the terminal DC differentiation by various stimuli. In contrast, MAC showed a high constitutive decysin mRNA expression, but expressed no MADDAM mRNA. On protein level similar results of MADDAM expression were obtained. Stimulation of MAC by LPS did not induce MADDAM mRNA expression, while decysin mRNA expression was strongly increased. Further investigations revealed that the well-known inducer of MAC differentiation, 1alpha,25-dihydroxyvitamin D3 up-regulated decysin mRNA expression during the differentiation of primary monocytes and myelomonocytic THP-1 cells into MAC. In vivo decysin mRNA expression was only detected in human colon, but not in other tissues we examined. Accordingly, isolated intestinal MAC expressed decysin mRNA. In conclusion, decysin and MADDAM mRNA expression were regulated in an opposite way during monocyte differentiation: MADDAM mRNA and protein was mainly detected in DC, whereas decysin mRNA expression was mainly found in MAC. Therefore only MADDAM, but not decysin is a suitable marker for human monocyte-derived DC.

Inverse regulation of the ADAM-family members, decysin and MADDAM/ADAM19 during monocyte differentiation

Two members of the ADAM (a disintegrin and metalloprotease)-family, MADDAM and decysin, were described as dendritic cell (DC) maturation markers. We are interested in monocyte differentiation and investigated in particular the expression pattern of both genes during the differentiation of human monocytes into DC and macrophages (MAC). Both genes are weakly expressed in freshly isolated monocytes. In immature DC decysin mRNA was absent, even after induction of the terminal differentiation of DC by CD40L or tumour necrosis factor-alpha (TNF-alpha). Only in DC maturated by lipopolysaccharide (LPS) strong signals of decysin mRNA were detected. However, MADDAM mRNA was expressed in immature DC and the expression was markedly increased after induction of the terminal DC differentiation by various stimuli. In contrast, MAC showed a high constitutive decysin mRNA expression, but expressed no MADDAM mRNA. On protein level similar results of MADDAM expression were obtained. Stimulation of MAC by LPS did not induce MADDAM mRNA expression, while decysin mRNA expression was strongly increased. Further investigations revealed that the well-known inducer of MAC differentiation, 1alpha,25-dihydroxyvitamin D3 up-regulated decysin mRNA expression during the differentiation of primary monocytes and myelomonocytic THP-1 cells into MAC. In vivo decysin mRNA expression was only detected in human colon, but not in other tissues we examined. Accordingly, isolated intestinal MAC expressed decysin mRNA. In conclusion, decysin and MADDAM mRNA expression were regulated in an opposite way during monocyte differentiation: MADDAM mRNA and protein was mainly detected in DC, whereas decysin mRNA expression was mainly found in MAC. Therefore only MADDAM, but not decysin is a suitable marker for human monocyte-derived DC.

Isolation and transplantation of allogeneic pulmonary endothelium derived from GFP transgenic mice

The isolation of primary endothelial cells from murine tissues has long been a challenge and remains a difficult task. Using GFP transgenic C57/BL6 mice as donors, we describe a reliable method to isolate pulmonary endothelial cells by flow cytometry after staining with DiI-Ac-low density lipoprotein (LDL). After mechanical dissociation of murine lung tissue and enzymatic digestion, adherent cells can be quickly stained and sorted by flow cytometry. The isolated cells express endothelial cell markers such as CD31, MECA32 and CD106 and stained positive for Isolectin B4. After 50-fold expansion using standard endothelial growth media, cells could be transplanted into lethally irradiated allogeneic hosts and were detectable using fluorescence microscopy up to 24-h post-transplantation in pulmonary tissue.

Hybrid Cell Vaccination in Metastatic Melanoma

Hybrid cell vaccination with cell fusion products (CFPs) of autologous tumor cells and mature allogenic MHC II bearing dendritic cells has been described to induce cytotoxic T lymphocyte (CTL)-mediated immune responses. The aim of this study was to assess safety, antitumor activity, and immune responses of a CFP-vaccine in patients with disseminated malignant melanoma. In a phase I/II study, we treated 11 patients by monthly intracutaneous or subcutaneous application of a CFP vaccine generated by electrofusion of autologous melanoma cells with mature allogenic dendritic cells. In addition, patients received subcutaneous low-dose interleukin-2 injections for 6 days after each vaccination. No serious adverse effects were observed. Ten patients showed progressive disease and one patient had a short-lasting stable disease. None of the patients developed a positive delayed-type hypersensitivity reaction against irradiated autologous melanoma cells. In 2 patients, who were monitored in more detail, we found no evidence of induction of a specific antimelanoma T-cell response by analyzing the proliferation, cytokine secretion, and cytotoxicity of their T cells toward autologous melanoma cells. No unequivocal beneficial effects of the used CFP vaccine could be demonstrated.

Pioglitazone and rofecoxib combined with angiostatically scheduled trofosfamide in the treatment of far-advanced melanoma and soft tissue sarcoma

BACKGROUND

Combined treatment approaches targeting tumor cells as well as stromal cells may control chemorefractory malignancies. In the current study, the authors sought to test one such combined approach in the treatment of chemorefractory melanoma and soft tissue sarcoma.

METHODS

A Phase II trial was initiated to analyze the activity of a continuously administered molecularly targeted treatment regimen (daily pioglitazone [45 mg administered orally] and rofecoxib [25 mg administered orally]) combined with sequentially added angiostatic chemotherapy for patients with previously treated metastatic melanoma (n = 19) or soft tissue sarcoma (n = 21). Angiostatic chemotherapy consisted of trofosfamide (50 mg) administered orally 3 times daily beginning on the 15th day after the start of molecularly targeted therapy.

RESULTS

Forty patients were evaluable for response and toxicity. Major side effects (World Health Organization Grade 3 or 4) were not observed. Objective responses and disease stabilization lasting longer than 6 months were noted in 11% and 11%, respectively, of all patients with melanoma and in 19% and 14%, respectively, of all patients with soft tissue sarcoma. Complete remission was noted in one patient with melanoma and in three patients with sarcoma. Both normal C-reactive protein (CRP) levels and CRP levels that decreased by > 30% during the 14-day biomodulator pretreatment period were found to be predictive of prolonged progression-free survival.

CONCLUSIONS

To our knowledge, the current study is the first to demonstrate that a novel, completely orally administered combined biomodulator/metronomic chemotherapy regimen may be active and well tolerated in patients with chemorefractory malignancies.

LPS resistance in monocytic cells caused by reverse signaling through transmembrane TNF (mTNF) is mediated by the MAPK/ERK pathway

The transmembrane form of tumor necrosis factor (mTNF), expressed on activated monocytes (MO) and macrophages (MPhi), is able to induce apoptosis in human endothelial cells (EC). Apoptosis is mediated by two distinct mechanisms: direct cell contact and a yet-unidentified soluble protein, death factor X. In addition, mTNF acts as a receptor that transduces a "reverse signal" into MO/MPhi when bound to the TNF receptor on EC. Reverse signaling by mTNF confers resistance to bacterial lipopolysaccharide (LPS). Stimulation of reverse signaling by mTNF blocks the ability of MO/MPhi to produce death factor X and proinflammatory cytokines. We have investigated which signaling pathways are used by mTNF acting as receptor. Reverse signaling triggers two independent pathways that can be distinguished by protein kinase C (PKC) inhibitors. The suppression of LPS-induced death factor X is dependent on PKC, whereas the suppression of LPS-mediated cytokine release is not. LPS and reverse signaling stimulate the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway. It is interesting that the activation of reverse signaling by mTNF renders MO/MPhi refractory to a subsequent activation of the MAPK/ERK pathway by LPS. Thus, reverse signaling achieves LPS resistance in monocytic cells through interference with key signal-transduction pathways.

Transcriptional regulation of CHI3L1, a marker gene for late stages of macrophage differentiation

The protein product of the CHI3L1 gene, human cartilage 39-kDa glycoprotein (HC-gp39), is a tissue-restricted, chitin-binding lectin and member of glycosyl hydrolase family 18. In contrast to many other monocyte/macrophage markers, its expression is absent in monocytes and strongly induced during late stages of human macrophage differentiation. To gain insights into the molecular mechanisms underlying its cell type-restricted and maturation-associated expression in macrophages, we initiated a detailed study of the proximal HC-gp39 promoter. Deletion analysis of reporter constructs in macrophage-like THP-1 cells localized a region directing high levels of macrophage-specific reporter gene expression to approximately 300 bp adjacent to the major transcriptional start site. The promoter sequence contained consensus binding sites for several known factors, and specific binding of nuclear PU.1, Sp1, Sp3, USF, AML-1, and C/EBP proteins was detectable in gel shift assays. In vivo footprinting assays with dimethyl sulfate demonstrate that the protection of corresponding sequences was enhanced in macrophages compared with monocytes. Mutational analysis of transcription factor binding sites indicated a predominant role for a single Sp1 binding site in regulating HC-gp39 promoter activity. In addition, gel shift assays using nuclear extracts of monocytes and macrophages demonstrated that the binding of nuclear Sp1, but not Sp3, markedly increases during macrophage differentiation. Our results further highlight the important role of Sp1 in macrophage gene regulation.

Regulation of 25-hydroxyvitamin D3-1 alpha-hydroxylase and production of 1 alpha,25-dihydroxyvitamin D3 by human dendritic cells

25-Hydroxyvitamin D3-1 alpha-hydroxylase (25(OH)D3-1 alpha-hydroxylase), the key enzyme of 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) production, is expressed in monocyte-derived macrophages (MACs). Here we show for the first time constitutive expression of 25(OH)D3-1 alpha-hydroxylase in monocyte-derived dendritic cells (DCs), which was increased after stimulation with lipopolysaccharide (LPS). Accordingly, DCs showed low constitutive production of 1,25(OH)2D3, but activation by LPS increased 1,25(OH)2D3 synthesis. In addition, 25(OH)D3-1 alpha-hydroxylase expression was found in blood DCs but not in CD34+-derived DCs. Next we analyzed the functional consequences of these results. Addition of 1,25(OH)2D3 at concentrations comparable with those produced by DCs inhibited the allostimulatory potential of DCs during the early phase of DC differentiation. However, terminal differentiation decreased the responsiveness of DCs to 1,25(OH)2D3. In conclusion, DCs are able to produce 1,25(OH)2D3 especially following stimulation with LPS. Terminal maturation renders DCs unresponsive to the effects of 1,25(OH)2D3, but those cells are able to suppress the differentiation of their own precursor cells in a paracrine way through the production of 1,25(OH)2D3.