A Phase 1, randomized, double-blind, placebo-controlled, single- and multiple-dose escalation study to evaluate the safety and pharmacokinetics/pharmacodynamics of PF-06835375, a C-X-C chemokine receptor type 5 directed antibody, in patients with systemic lupus erythematosus or rheumatoid arthritis.

BACKGROUND: The objective of this study was to evaluate the safety, tolerability, pharmacokinetics, and pharmacodynamics of PF­06835375, a potent selective afucosyl immunoglobulin G1 antibody targeting C-X-C chemokine receptor type 5 (CXCR5) that potentially depletes B cells, follicular T helper (Tfh) cells, and circulating Tfh-like (cTfh) cells, in patients with systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). METHODS: This first-in-human, multicenter, double-blind, sponsor-open, placebo-controlled Phase 1 study recruited patients aged 18-70 years with SLE or RA. In Part A, patients received single doses of intravenous PF-06835375 (dose range: 0.03-6 mg) or placebo in six sequential single ascending dose (SAD) cohorts. In Part B, patients received repeat doses of subcutaneous PF-06835375 (dose range: 0.3-10 mg) or placebo on Days 1 and 29 in five multiple ascending dose (MAD) cohorts. Tetanus/Diphtheria (Td) and Meningococcal B (MenB/Trumenba™) vaccines were administered at Day 4 (Td and MenB) and Week 8 (MenB only) to assess PF-06835375 functional effects. Endpoints included treatment-emergent adverse events (TEAEs), pharmacokinetic parameters, pharmacodynamic effects on B and cTfh cells, and biomarker counts, vaccine response, and exploratory differential gene expression analysis. Safety, pharmacokinetic, and pharmacodynamic endpoints are summarized descriptively. The change from baseline of B and Tfh cell-specific genes over time was calculated using a prespecified mixed-effects model, with a false discovery rate < 0.05 considered statistically significant. RESULTS: In total, 73 patients were treated (SAD cohorts: SLE, n = 17; RA, n = 14; MAD cohorts: SLE, n = 22; RA, n = 20). Mean age was 53.3 years. Sixty-two (84.9%) patients experienced TEAEs (placebo n = 17; PF-06835375 n = 45); most were mild or moderate. Three (9.7%) patients experienced serious adverse events. Mean t1/2 ranged from 3.4-121.4 h (SAD cohorts) and 162.0-234.0 h (MAD cohorts, Day 29). B and cTfh cell counts generally showed dose-dependent reductions across cohorts (range of mean maximum depletion: 67.3-99.3%/62.4-98.7% [SAD] and 91.1-99.6%/89.5-98.1% [MAD], respectively). B cell-related genes and pathways were significantly downregulated in patients treated with PF-06835375. CONCLUSIONS: These data support further development of PF-06835375 to assess the clinical potential for B and Tfh cell depletion as a treatment for autoimmune diseases. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT03334851.

Improvements in immune/melanocyte biomarkers with JAK3/TEC family kinase inhibitor ritlecitinib in vitiligo.

BACKGROUND: Vitiligo is an autoimmune depigmenting disorder with no effective and safe treatments. Its pathogenesis is not fully elucidated. OBJECTIVE: This substudy of a randomized, double-blind, placebo-controlled phase 2b trial (NCT03715829) evaluated effects of ritlecitinib, an oral JAK3/TEC family kinase inhibitor, on skin and blood biomarkers in participants with nonsegmental vitiligo (NSV). METHODS: Sixty-five adults with NSV participated in the substudy and received daily treatment for 24 weeks with placebo (n = 14) or ritlecitinib with or without a 4-week loading dose: 200 (loading dose)/50 mg (n = 13), 100/50 mg (n = 12), 50 mg (n = 11), 30 mg (n = 8), or 10 mg (n = 6). Skin (lesional and nonlesional) biopsy samples were obtained at baseline and at 4 and 24 weeks. Changes from baseline to weeks 4 and 24 in skin and blood molecular and cellular biomarkers were evaluated by RNA sequencing, quantitative real-time PCR, proteomic analysis, and flow cytometry. RESULTS: Ritlecitinib-treated groups showed downregulation of immune biomarkers and upregulation of melanocyte-related markers at weeks 4 and 24 compared to baseline and/or placebo. Significant reductions were seen in CD3+/CD8+ T-cell infiltrates, with significant increases in melanocyte markers (tyrosinase; Melan-A) in NSV lesions in the 50 mg ritlecitinib groups (both P < .05). There was significant, dose-dependent downregulation in T-cell activation, NK, cytotoxic, and regulatory markers in lesional skin (IL-2, IL2-RA, IL-15, CCR7, CD5, CRTAM, NCR1, XCL1, KIR3DL1, FASLG, KLRD; P < .05). TH1 and TH2 markers were also downregulated in lesional skin and blood in a dose-dependent manner (P < .05). Changes in immune biomarkers correlated with clinical response. CONCLUSIONS: Ritlecitinib significantly downregulated proinflammatory biomarkers and increased melanocyte products in skin and blood of participants with NSV, suggesting its potential in treatment. Ritlecitinib-mediated changes positively correlated with clinical response.

Antitumor Necrosis Factor-like Ligand 1A Therapy Targets Tissue Inflammation and Fibrosis Pathways and Reduces Gut Pathobionts in Ulcerative Colitis.

BACKGROUND: The first-in-class treatment PF-06480605 targets the tumor necrosis factor-like ligand 1A (TL1A) molecule in humans. Results from the phase 2a TUSCANY trial highlighted the safety and efficacy of PF-06480605 in ulcerative colitis. Preclinical and in vitro models have identified a role for TL1A in both innate and adaptive immune responses, but the mechanisms underlying the efficacy of anti-TL1A treatment in inflammatory bowel disease (IBD) are not known. METHODS: Here, we provide analysis of tissue transcriptomic, peripheral blood proteomic, and fecal metagenomic data from the recently completed phase 2a TUSCANY trial and demonstrate endoscopic improvement post-treatment with PF-06480605 in participants with ulcerative colitis. RESULTS: Our results revealed robust TL1A target engagement in colonic tissue and a distinct colonic transcriptional response reflecting a reduction in inflammatory T helper 17 cell, macrophage, and fibrosis pathways in patients with endoscopic improvement. Proteomic analysis of peripheral blood revealed a corresponding decrease in inflammatory T-cell cytokines. Finally, microbiome analysis showed significant changes in IBD-associated pathobionts, Streptococcus salivarius, S. parasanguinis, and Haemophilus parainfluenzae post-therapy. CONCLUSIONS: The ability of PF-06480605 to engage and inhibit colonic TL1A, targeting inflammatory T cell and fibrosis pathways, provides the first-in-human mechanistic data to guide anti-TL1A therapy for the treatment of IBD.

Anti-TL1A Antibody PF-06480605 Safety and Efficacy for Ulcerative Colitis: A Phase 2a Single-Arm Study.

BACKGROUND & AIMS: An immune component of inflammatory bowel disease is up-regulated tumor necrosis factor-like ligand 1A (TL1A). Anti-TL1A antibodies such as PF-06480605, a fully human immunoglobulin G1 monoclonal antibody, may have therapeutic potential. METHODS: This Phase 2a, multicenter, single-arm, open-label study (TUSCANY) evaluated safety, tolerability, efficacy, pharmacokinetics, and immunogenicity in PF-06480605-treated participants with moderate to severe ulcerative colitis (UC). Participants received 500 mg intravenous PF-06480605 every 2 weeks, 7 doses total, with a 3-month follow-up period. Primary safety and efficacy endpoints were the incidence of adverse events (AEs) and week 14 endoscopic improvement (EI) (Mayo endoscopic subscore = 0 or 1), respectively. Secondary endpoints included total soluble TL1A (free/drug-bound) (sTL1A), incidence of anti-drug and neutralizing antibodies, PF-06480605 concentrations, and changes in fecal calprotectin and high-sensitivity C-reactive protein. Histology was assessed at week 14. RESULTS: The study enrolled 50 participants; 42 completed. Of 109 treatment-emergent AEs, 18 were treatment-related. The most common AEs were UC disease exacerbation and arthralgia (6 participants each). Four serious AEs, no deaths, and no malignancies were reported. Week 14 EI was observed in a statistically significant proportion of participants (38.2% [uniformly minimum-variance unbiased estimator, per protocol population]). Minimal histologic disease was observed after treatment (Robarts Histopathology Index ≤5: 33.3%; Geboes Index ≤3.2: 47.6%). sTL1A increase over time from baseline indicated sustained target engagement. Forty-one participants (82%) tested positive for anti-drug antibodies and 5 (10%) for neutralizing antibodies. CONCLUSIONS: PF-06480605 demonstrated an acceptable safety profile and statistically significant EI in participants with moderate to severe UC, warranting further study in a larger participant cohort. Tissue histopathology analyses support this conclusion. TRIAL REGISTRATION NUMBER: https://clinicaltrials.gov/NCT02840721.

Anti-MAdCAM-1 antibody (PF-00547659) for active refractory Crohn's disease in Japanese and Korean patients: the OPERA study.

BACKGROUND/AIMS: PF-00547659 is a monoclonal antibody against human mucosal addressin cell adhesion molecule-1 (MAdCAM-1) that prevents the binding of α4ß7+ lymphocytes to MAdCAM-expressing sites in the gastrointestinal tract with high affinity and selectivity, and is being developed for the treatment of Crohn's disease (CD). METHODS: OPERA is a randomized, multicenter, double-blind, placebo-controlled study to investigate the efficacy, safety, and pharmacokinetics of PF-00547659 following subcutaneous administration in subjects with active CD, a history of failure or intolerance to anti-tumor necrosis factor and/or immunosuppressants, high-sensitivity C-reactive protein > 3.0 mg/L, and ulcers on colonoscopy. The primary endpoint was Crohn's Disease Activity Index-70 response at week 8 or 12. Subpopulation analyses for Asian subjects were performed as some differences are observed in genetics and clinical phenotypes in Asian CD patients compared with Western patients. RESULTS: In this study, 265 CD subjects were randomized, with a subpopulation of 21 subjects (8 Japanese and 13 Korean) defined as the Asian population. In the overall and Asian populations; PF-00547659 was pharmacologically active as evidenced by soluble MAdCAM and circulating ß7+ central memory CD4+ T-lymphocytes, although no clear evidence of efficacy was observed in any clinical endpoints; pharmacokinetics of PF-00547659 in the Asian subpopulation was generally comparable to the overall population; and the safety profile of PF-00547659 appeared acceptable up to 12 weeks of treatment. CONCLUSIONS: In the overall and Asian populations, efficacy of PF-00547659 could not be demonstrated using any clinical endpoints compared with placebo. Pharmacokinetics and safety of PF-00547659 were generally comparable. Further studies with larger numbers of patients are required to confirm our results. (Trial Registration Number: NCT01276509).

First-in-human, randomized dose-escalation study of the safety, tolerability, pharmacokinetics, pharmacodynamics and immunogenicity of PF-06480605 in healthy subjects.

AIMS: Human genetic, tissue expression, proteomics, transcriptomics and nonclinical studies implicate tumour necrosis factor α-like ligand 1A (TL1A) as a novel target in inflammatory bowel disease (IBD). PF-06480605, a fully human immunoglobulin G1 monoclonal antibody, targets TL1A. This first-in-human, Phase 1, dose-escalation study assessed safety, tolerability, pharmacokinetics, pharmacodynamics and immunogenicity of intravenous (IV) and subcutaneous (SC) PF-06480605 in healthy subjects (NCT01989143). METHODS: Ninety-two subjects were randomized to single ascending doses (SAD), PF-06480605 1 mg, 3 mg, 10 mg, 30 mg, 100 mg, 300 mg, 600 mg or 800 mg IV, or multiple ascending doses (MAD), PF-06480605 3 × 500 mg IV, or 3 × 30 mg, 3 × 100 mg, or 3 × 300 mg SC every 2 weeks for three doses, or placebo. Safety, tolerability, pharmacokinetics, immunogenicity profiles and total TL1A, anti-drug antibody (ADA) and neutralizing antibody (NAb) levels were assessed at pre-determined times. RESULTS: PF-06480605 SAD up to 800 mg IV and MAD up to 300 mg ×3 SC and 500 mg ×3 IV were well tolerated. Overall, there were 45 and 44 treatment-emergent adverse events in SAD and MAD cohorts, respectively, and no deaths or serious adverse events. PF-06480605 exposure generally increased dose-dependently. ADA and NAb levels did not impact safety, pharmacokinetics, or pharmacodynamics at higher doses. Target engagement was demonstrated through dose-dependent differences in serum total soluble TL1A concentrations for PF-06480605 vs placebo cohorts. CONCLUSIONS: PF-06480605 was generally well tolerated, and binding of soluble TL1A was maintained throughout the dose interval, supporting further study of PF-06480605 in patients with IBD and other inflammatory conditions.

Molecular Profiling of Ulcerative Colitis Subjects from the TURANDOT Trial Reveals Novel Pharmacodynamic/Efficacy Biomarkers.

BACKGROUND AND AIMS: To define pharmacodynamic and efficacy biomarkers in ulcerative colitis [UC] patients treated with PF-00547659, an anti-human mucosal addressin cell adhesion molecule-1 [MAdCAM-1] monoclonal antibody, in the TURANDOT study. METHODS: Transcriptome, proteome and immunohistochemistry data were generated in peripheral blood and intestinal biopsies from 357 subjects in the TURANDOT study. RESULTS: In peripheral blood, C-C motif chemokine receptor 9 [CCR9] gene expression demonstrated a dose-dependent increase relative to placebo, but in inflamed intestinal biopsies CCR9 gene expression decreased with increasing PF-00547659 dose. Statistical models incorporating the full RNA transcriptome in inflamed intestinal biopsies showed significant ability to assess response and remission status. Oncostatin M [OSM] gene expression in inflamed intestinal biopsies demonstrated significant associations with, and good accuracy for, efficacy, and this observation was confirmed in independent published studies in which UC patients were treated with infliximab or vedolizumab. Compared with the placebo group, intestinal T-regulatory cells demonstrated a significant increase in the intermediate 22.5-mg dose cohort, but not in the 225-mg cohort. CONCLUSIONS: CCR9 and OSM are implicated as novel pharmacodynamic and efficacy biomarkers. These findings occur amid coordinated transcriptional changes that enable the definition of surrogate efficacy biomarkers based on inflamed biopsy or blood transcriptomics data.ClinicalTrials.gov identifierNCT01620255.

Effect of PF-00547659 on Central Nervous System Immune Surveillance and Circulating ß7+ T Cells in Crohn's Disease: Report of the TOSCA Study.

BACKGROUND AND AIMS: Progressive multifocal leukoencephalopathy [PML], a brain infection associated with anti-integrin drugs that inhibit lymphocyte translocation from bloodstream to tissue, can be fatal. Decreased central nervous system [CNS] immune surveillance leading to this infection has been reported in patients with multiple sclerosis or Crohn's disease treated with anti-integrin antibody natalizumab. PF-00547659 is an investigational human monoclonal antibody for inflammatory bowel disease, targeted against α4ß7-mucosal addressin cell-adhesion molecule-1 [the integrin ligand selectively expressed in the gut]. We hypothesised that this selective agent would not affect central nervous system immune surveillance. METHODS: Cerebrospinal fluid from five healthy volunteers, and from 10 patients with Crohn's disease previously treated with immunosuppressants, was evaluated to assess the feasibility of the study. Subsequently, 39 patients with active Crohn's disease and previous immunosuppression were evaluated over 12 weeks of PF-00547659-induction therapy. We measured total lymphocytes, T cell subsets in cerebrospinal fluid, and circulating ß7+ memory cells. Disease activity was assessed using the Harvey-Bradshaw Index. RESULTS: Patients treated with PF-00547659 had no reduction of cerebrospinal fluid lymphocytes, T-lymphocyte subsets, or CD4:CD8 ratio, whereas circulating ß7+ memory cells increased significantly. A total of 28/35 [80%] patients had a clinical response and 27/34 [79%] had disease remission. Treatment-related adverse events, none serious, were reported in 23/49 [47%] patients. CONCLUSIONS: In patients with active Crohn's disease, natalizumab therapy increases the risk for PML, and the increased risk is thought to be associated with iatrogenic leukopenia within the CNS. PML under PF-00547659 may be a lesser concern, as this agent did not reduce lymphocytes or T cell subsets in the cerebrospinal fluid.

Anti-MAdCAM Antibody Increases ß7+ T Cells and CCR9 Gene Expression in the Peripheral Blood of Patients With Crohn's Disease.

OBJECTIVE: To define pharmacodynamic biomarkers in the peripheral blood of patients with Crohn's disease [CD] after treatment with PF-00547659, an anti-human mucosal addressin cell adhesion molecule-1 [MAdCAM-1] monoclonal antibody. METHODS: In this Phase 2, randomised, double-blind, controlled study [OPERA], blood samples were analysed from patients with moderate to severe active CD who received placebo or 22.5 mg, 75 mg, or 225 mg of PF-00547659 subcutaneously at baseline and at Weeks 4 and 8, with follow-up at Week 12. Soluble MAdCAM [sMAdCAM] was measured by mass spectrometry, ß7-expressing T cells by flow cytometry, and gene transcriptome by RNA sequencing. RESULTS: A slight increase in sMAdCAM was measured in the placebo group from baseline to Week 12 [6%], compared with significant decreases in all PF-00547659 groups [-87% to -98%]. A slight increase from baseline to Week 12 was observed in frequency and molecules of equivalent soluble fluorochrome for ß7+ central memory T cells in the placebo group [4%], versus statistically significant increases in the active treatment groups [48% to 81%]. Similar trends were seen for ß7+ effector memory T cells [placebo, 8%; PF-00547659, 84-138%] and ß7+ naïve T cells [8%; 13-50%]. CCR9 gene expression had statistically significant up-regulation [p = 1.09e-06; false discovery rate < 0.1] with PF-00547659 treatment, and was associated with an increase in ß7+ T cells. CONCLUSIONS: Results of the OPERA study demonstrate positive pharmacology and dose-dependent changes in pharmacodynamic biomarker measurements in blood, including changes in cellular composition of lymphocytes and corresponding CCR9 gene expression changes.

Phase II evaluation of anti-MAdCAM antibody PF-00547659 in the treatment of Crohn's disease: report of the OPERA study.

OBJECTIVE: This phase II, randomised, double-blind, placebo-controlled clinical trial was designed to evaluate the efficacy and safety of PF-00547659, a fully human monoclonal antibody that binds to human mucosal addressin cell adhesion molecule (MAdCAM) to selectively reduce lymphocyte homing to the intestinal tract, in patients with moderate-to-severe Crohn's disease (CD). DESIGN: Eligible adults were aged 18-75 years, with active moderate-to-severe CD (Crohn's Disease Activity Index (CDAI) 220-450), a history of failure or intolerance to antitumour necrosis factor and/or immunosuppressive agents, high-sensitivity C reactive protein >3.0 mg/L and ulcers on colonoscopy. Patients were randomised to PF-00547659 22.5 mg, 75 mg or 225 mg or placebo. The primary endpoint was CDAI 70-point decrease from baseline (CDAI-70) at week 8 or 12. RESULTS: In all, 265 patients were eligible for study entry. Although CDAI-70 response was not significantly different with placebo versus PF-00547659 treatment at weeks 8 or 12, remission rate was greater in patients with higher baseline C reactive protein (>5 mg/L vs >18.8 mg/L, respectively). Soluble MAdCAM decreased significantly from baseline to week 2 in a dose-related manner and remained low during the study in PF-00547659-treated patients. Circulating ß7+ CD4+ central memory T-lymphocytes increased at weeks 8 and 12 with PF-00547659 treatment. No safety signal was seen. CONCLUSIONS: Clinical endpoint differences between PF-00547659 and placebo did not reach statistical significance in patients with moderate-to-severe CD. PF-00547659 was pharmacologically active, as shown by a sustained dose-related decrease in soluble MAdCAM and a dose-related increase in circulating ß7+ central memory T cells. TRIAL REGISTRATION NUMBER: NCT01276509; Results.

Anti-MAdCAM antibody (PF-00547659) for ulcerative colitis (TURANDOT): a phase 2, randomised, double-blind, placebo-controlled trial.

BACKGROUND: PF-00547659 is a fully human monoclonal antibody that binds to human mucosal addressin cell adhesion molecule-1 (MAdCAM-1) to selectively reduce lymphocyte homing to the intestinal tract. We aimed to assess the efficacy and safety of PF-00547659 in patients with moderate to severe ulcerative colitis. METHODS: This phase 2, randomised, double-blind, placebo-controlled clinical trial recruited patients aged 18-65 years from 105 centres in 21 countries, with a history (≥3 months) of active ulcerative colitis extending more than 15 cm beyond the anal verge (with a total Mayo score ≥6 and a Mayo endoscopic subscore ≥2) who had failed or were intolerant to at least one conventional therapy. Patients were stratified by previous anti-TNFα treatment, and randomly assigned by a computer-generated randomisation schedule to receive a subcutaneous injection of 7·5 mg, 22·5 mg, 75 mg, or 225 mg PF-00547659 or placebo at baseline, then every 4 weeks. Patients, investigators, and sponsors were blinded to the treatment. The primary endpoint was the proportion of patients achieving remission (total Mayo score ≤2 with no individual subscore >1 and rectal bleeding subscore ≤1) at week 12. The efficacy analysis included all patients who received at least one dose of the randomised treatment; the safety analysis was done according to treatment received. All p values were one-sided and multiplicity-adjusted. This study is registered with ClinicalTrials.gov, number NCT01620255. FINDINGS: Between Nov 2, 2012, and Feb 4, 2016, we screened 587 patients; 357 were eligible and randomly assigned to receive placebo (n=73) or PF-00547659 at doses of 7·5 mg (n=71), 22·5 mg (n=72), 75 mg (n=71), or 225 mg (n=70). Remission rates at week 12 were significantly greater in three of four active-treatment groups than in the placebo group (2·7% [two of 73]): 7·5 mg (11·3% [eight of 71]), 22·5 mg (16·7% [12 of 72]), 75 mg (15·5% [11 of 71]), and 225 mg (5·7% [four of 70]). These rates corresponded to a stratum-adjusted (anti-TNFα-naive and anti-TNFα-experienced) risk difference versus placebo of 8·0% for 7·5 mg (90% CI 1·9 to 14, p=0·0425), 12·8% for 22·5 mg (5·6 to 19·9, p=0·0099), 11·8% for 75 mg (4·8 to 18·8, p=0·0119), and 2·6% for 225 mg (-1·2 to 6·4, p=0·1803). Four of 73 (5·5%) patients had a serious adverse event in the placebo group, ten of 71 (14·1%) in the 7·5 mg group, one of 70 (1·4%) in the 22·5 mg group, three of 73 (4·1%) in the 75 mg group, and three of 70 (4·3%) in the 225 mg group. No safety signal was observed for the study drug. INTERPRETATION: PF-00547659 was safe and well tolerated in this patient population, and better than placebo for induction of remission in patients with moderate to severe ulcerative colitis. The greatest clinical effects were observed with the 22·5 mg and 75 mg doses. FUNDING: Pfizer.

Heat shock protein-mediated cross-presentation of exogenous HIV antigen on HLA class I and class II.

Strong CD4(+) and CD8(+) T cell responses are considered important immune components for controlling HIV infection, and their priming may be central to an effective HIV vaccine. We describe in this study an approach by which multiple CD4(+) and CD8(+) T cell epitopes are processed and presented from an exogenously added HIV-1 Gag-p24 peptide of 32 aa complexed to heat shock protein (HSP) gp96. CD8(+) T cell recognition of the HSP/peptide complex, but not the peptide alone, was inhibited by brefeldin A, suggesting an endoplasmic reticulum-dependent pathway. This is the first report to describe efficient processing and simultaneous presentation of overlapping class I- and class II-restricted epitopes from the same extracellularly added precursor peptide complexed to HSP. Given previous reports of the strong immunogenicity of HSP/peptide complexes, the present data suggest that HSP-complexed peptides containing multiple MHC class I- and class II-restricted epitopes represent potential vaccine candidates for HIV and other viral infections suitable to induce effective CTL memory by simultaneously providing CD4 T cell help.

Bone marrow-derived versus parenchymal sources of inducible nitric oxide synthase in experimental autoimmune encephalomyelitis.

The role of nitric oxide (NO) in central nervous system (CNS) inflammation is uncertain. Whereas experimental autoimmune encephalomyelitis (EAE) is exacerbated in mice deficient in inducible nitric oxide synthase (iNOS), inhibitor studies have suggested a pro-inflammatory role for NO. These discrepancies may reflect balance between immunoregulatory and neurocytopathologic roles for NO. We investigated selective effects of bone marrow-derived versus CNS parenchymal sources of iNOS in EAE in chimeric mice. Chimeras that selectively expressed or ablated iNOS in leukocytes both showed significant delay in disease onset, with no difference in disease severity. We conclude that bone marrow-derived and CNS parenchymal sources of iNOS-derived NO both play a regulatory role in EAE.