Foxp3+ regulatory T (Treg) cells suppress different types of immune responses to help maintain homeostasis in the body. How Treg cells regulate humoral immunity, including germinal center reactions, is unclear. Here we identify a subset of Treg cells expressing CXCR5 and Bcl6, and localized in the germinal centers in mouse as well as human. The expression of CXCR5 on Treg cells depends on Bcl6. These CXCR5+Bcl6+ Treg cells are absent in thymus but can be de novo generated from CXCR5-Foxp3+ natural Treg precursors. Lack of CXCR5+ Treg cells leads to greater germinal center reactions. These results unveil a Bcl6-CXCR5 axis in Treg cells that undermines the development of follicular regulatory T (Tfr) cells that function to inhibit the germinal center reaction.

In immune responses, activated T cells migrate to B cell follicles and develop to T follicular helper (Tfh) cells, a new subset of CD4+ T cells specialized in providing help to B lymphocytes in the induction of germinal centers 1,2. Although Bcl6 has been shown to be essential in Tfh cell function, it may not regulate the initial migration of T cells 3 or the induction of Tfh program as exampled by C-X-C chemokine receptor type 5 (CXCR5) upregulation 4. Here, we show that Achaete-Scute homologue 2 (Ascl2), a basic helix-loop-helix (bHLH) transcription factor 5, is selectively upregulated in its expression in Tfh cells. Ectopic expression of Ascl2 upregulates CXCR5 but not Bcl6 and downregulates C-C chemokine receptor 7 (CCR7) expression in T cells in vitro and accelerates T cell migration to the follicles and Tfh cell development in vivo. Genome-wide analysis indicates that Ascl2 directly regulates Tfh-related genes while inhibits expression of Th1 and Th17 genes. Acute deletion of Ascl2 as well as blockade of its function with the Id3 protein in CD4+ T cells results in impaired Tfh cell development and the germinal center response. Conversely, mutation of Id3, known to cause antibody-mediated autoimmunity, greatly enhances Tfh cell generation. Thus, Ascl2 directly initiates Tfh cell development.

Background Endogenous or iatrogenic antitumor immune responses can improve the course of follicular lymphoma (FL), but may be diminished by immune checkpoints in the tumor microenvironment. These may include effects of programmed death (PD)-1, a co-inhibitory receptor that impairs T-cell function and is highly expressed on intratumoral T cells. In a Phase II trial, we determined the activity of pidilizumab, a humanized anti-PD-1 monoclonal antibody, with rituximab in patients with relapsed FL. Methods FL patients with rituximab-sensitive disease relapsing after 1–4 prior therapies were eligible. Pidilizumab was administered at 3 mg/kg every 4 weeks for 4 infusions, plus 8 optional infusions every 4 weeks for patients with stable disease or better. Starting 2 weeks after the first infusion of pidilizumab, rituximab was given at 375 mg/m2 weekly for 4 weeks. The primary endpoint was to assess the overall response rate. Analysis was by intention to treat. Peripheral blood and tumor biopsies were studied to assess immunological effects of pidilizumab. This trial has been completed and was registered at www.clinicaltrials.gov as NCT00904722. Findings The combination was well-tolerated, with no autoimmune or therapy-related grade 3/4 toxicities. The most common grade 1 adverse events were anemia (14 patients) and fatigue (13 patients), and the most common grade 2 adverse event was respiratory infection (5 patients). Overall 19/29 (66%) and complete 15/29 (52%) response rates in 29 evaluable patients were high, with tumor regression in 25/29 (86%) of patients. Median progression-free survival was 18.8 months (95% CI: 14.7 months to not reached). The median response duration for the 19 responders was 20.2 months (95% CI: 13.9 months to not reached). Correlative studies of blood and tumor provided insights into predicting response and understanding mechanisms involved. Interpretation Pidilizumab with rituximab is well-tolerated and its activity compared favorably to historical retreatment with rituximab monotherapy in patients with relapsed FL. Our results establish that immune checkpoint blockade is worthy of further study in FL. Funding National Institutes of Health, Leukemia and Lymphoma Society, Cure Tech Ltd, and UT MD Anderson Cancer Center.

In order to further the present knowledge of the emerging severe acute respiratory syndrome-associated coronavirus (SARS-CoV), 486 different specimens from 54 patients with a clinical diagnosis of SARS were investigated for the presence of viral RNA, and 314 plasma specimens of 73 patients were examined for IgM and IgG antibodies specific against SARS-CoV using an indirect ELISA. Viral RNA was detectable in 28 of the 54 patients tested. Cumulative data showed that 67 of the 73 SARS patients demonstrated seroconversion by week 5 of illness. In contrast, only 1 of 278 healthy subjects enrolled in the study was found to be positive for the IgG antibody. Coexistence of viral RNA in plasma and specific antibodies was simultaneously observed over three consecutive weeks in two critical cases. In three convalescent patients in particular, cultivable SARS-CoV was detected in stool or urine specimens for longer than 4 weeks (29-36 days). These findings suggest that SARS-CoV may remain viable in the excretions of convalescent patients.

The M protein of SARS-CoV holds dominant cellular immunogenicity. This, together with previous reports of a strong humoral response against the M protein, may help to further explain the immunogenicity of SARS and serves as potential targets for SARS-CoV vaccine design.