FCRL6 distinguishes mature cytotoxic lymphocytes and is upregulated in patients with B-cell chronic lymphocytic leukemia

Proteomics screening after pediatric allogenic hematopoietic stem cell transplantation reveals an association between increased expression of inhibitory receptor FCRL6 on γδ T cells and cytomegalovirus reactivation

We studied the associations between inflammation-related proteins in circulation and complications after pediatric allogenic hematopoietic stem cell transplantation (HSCT), to reveal proteomic signatures or individual soluble proteins associated with specific complications after HSCT. We used a proteomics method called Proximity Extension Assay to repeatedly measure 180 different proteins together with clinical variables, cellular immune reconstitution and blood viral copy numbers in 27 children (1-18 years of age) during a 2-year follow-up after allogenic HSCT. Protein profile analysis was performed using unsupervised hierarchical clustering and a regression-based method, while the Bonferroni-corrected Mann-Whitney U-test was used for time point-specific comparison of individual proteins against outcome. At 6 months after allogenic HSCT, we could identify a protein profile pattern associated with occurrence of the complications such as chronic graft-versus-host disease, viral infections, relapse and death. When protein markers were analyzed separately, the plasma concentration of the inhibitory and cytotoxic T-cell surface protein FCRL6 (Fc receptor-like 6) was higher in patients with cytomegalovirus (CMV) viremia [log2-fold change 1.5 (P = 0.00099), 2.5 (P = 0.00035) and 2.2 (P = 0.045) at time points 6, 12 and 24 months]. Flow cytometry confirmed that FCRL6 expression was higher in innate-like γδ T cells, indicating that these cells are involved in controlling CMV reactivation in HSCT recipients. In conclusion, the potentially druggable FCRL6 receptor on cytotoxic T cells appears to have a role in controlling CMV viremia after HSCT. Furthermore, our results suggest that system-level analysis is a useful addition to the studying of single biomarkers in allogenic HSCT.

FCRL1 immunoregulation in B cell development and malignancy

Immunotherapeutic targeting of surface regulatory proteins and pharmacologic inhibition of critical signaling pathways has dramatically shifted our approach to the care of individuals with B cell malignancies. This evolution in therapy reflects the central role of the B cell receptor (BCR) signaling complex and its co-receptors in the pathogenesis of B lineage leukemias and lymphomas. Members of the Fc receptor-like gene family (FCRL1-6) encode cell surface receptors with complex tyrosine-based regulation that are preferentially expressed by B cells. Among them, FCRL1 expression peaks on naïve and memory B cells and is unique in terms of its intracellular co-activation potential. Recent studies in human and mouse models indicate that FCRL1 contributes to the formation of the BCR signalosome, modulates B cell signaling, and promotes humoral responses. Progress in understanding its regulatory properties, along with evidence for its over-expression by mature B cell leukemias and lymphomas, collectively imply important yet unmet opportunities for FCRL1 in B cell development and transformation. Here we review recent advances in FCRL1 biology and highlight its emerging significance as a promising biomarker and therapeutic target in B cell lymphoproliferative disorders.

Identification by Exome Sequencing of Predisposing Variants in Familial Cases of Autoinflammatory Recurrent Fevers

Periodic fever syndromes include autoinflammatory disorders (AID) that involve innate immunity. These disorders are characterized by recurrent fevers and aberrant multi-organ inflammation, without any involvement of T or B cells or the presence of autoantibodies. A complex genetic architecture has been recognized for many AID. However, this complexity has only been partially uncovered for familial Mediterranean fever and other conditions that have a classical monogenic origin and Mendelian transmission. Several gene panels are currently available for molecular diagnosis in patients suspected of having AID. However, even when an extensive number of genes (up to 50-100) are tested in a cohort of clinically selected patients, the diagnostic yield of AID ranges between 15% and 25%, depending on the clinical criteria used for patient selection. In the remaining 75-85% of cases, it is conceivable that the causative gene or genes responsible for a specific condition are still elusive. In these cases, the disease could be explained by variants, either recessive or dominant, that have a major effect on unknown genes, or by the cumulative impact of different variants in more than one gene, each with minor additive effects. In this study, we focused our attention on five familial cases of AID presenting with classical autosomal dominant transmission. To identify the probable monogenic cause, we performed exome sequencing. Through prioritization, filtering, and segregation analysis, we identified a few variants for each family. Subsequent bioinformatics evaluation and pathway analysis helped to narrow down the best candidate genes for each family to FCRL6, PKN1, STAB1, PTDGR, and VCAM1. Future studies on larger cohorts of familial cases will help confirm the pathogenic role of these genes in the pathogenesis of these complex disorders.

Exposure of progressive immune dysfunction by SARS-CoV-2 mRNA vaccination in patients with chronic lymphocytic leukemia: A prospective cohort study

BACKGROUND

Patients with chronic lymphocytic leukemia (CLL) have reduced seroconversion rates and lower binding antibody (Ab) and neutralizing antibody (NAb) titers than healthy individuals following Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) mRNA vaccination. Here, we dissected vaccine-mediated humoral and cellular responses to understand the mechanisms underlying CLL-induced immune dysfunction.

METHODS AND FINDINGS

We performed a prospective observational study in SARS-CoV-2 infection-naïve CLL patients (n = 95) and healthy controls (n = 30) who were vaccinated between December 2020 and June 2021. Sixty-one CLL patients and 27 healthy controls received 2 doses of the Pfizer-BioNTech BNT162b2 vaccine, while 34 CLL patients and 3 healthy controls received 2 doses of the Moderna mRNA-1273 vaccine. The median time to analysis was 38 days (IQR, 27 to 83) for CLL patients and 36 days (IQR, 28 to 57) for healthy controls. Testing plasma samples for SARS-CoV-2 anti-spike and receptor-binding domain Abs by enzyme-linked immunosorbent assay (ELISA), we found that all healthy controls seroconverted to both antigens, while CLL patients had lower response rates (68% and 54%) as well as lower median titers (23-fold and 30-fold; both p < 0.001). Similarly, NAb responses against the then prevalent D614G and Delta SARS-CoV-2 variants were detected in 97% and 93% of controls, respectively, but in only 42% and 38% of CLL patients, who also exhibited >23-fold and >17-fold lower median NAb titers (both p < 0.001). Interestingly, 26% of CLL patients failed to develop NAbs but had high-titer binding Abs that preferentially reacted with the S2 subunit of the SARS-CoV-2 spike. Since these patients were also seropositive for endemic human coronaviruses (HCoVs), these responses likely reflect cross-reactive HCoV Abs rather than vaccine-induced de novo responses. CLL disease status, advanced Rai stage (III-IV), elevated serum beta-2 microglobulin levels (β2m >2.4 mg/L), prior therapy, anti-CD20 immunotherapy (<12 months), and intravenous immunoglobulin (IVIg) prophylaxis were all predictive of an inability to mount SARS-CoV-2 NAbs (all p ≤ 0.03). T cell response rates determined for a subset of participants were 2.8-fold lower for CLL patients compared to healthy controls (0.05, 95% CI 0.01 to 0.27, p < 0.001), with reduced intracellular IFNγ staining (p = 0.03) and effector polyfunctionality (p < 0.001) observed in CD4+ but not in CD8+ T cells. Surprisingly, in treatment-naïve CLL patients, BNT162b2 vaccination was identified as an independent negative risk factor for NAb generation (5.8, 95% CI 1.6 to 27, p = 0.006). CLL patients who received mRNA-1273 had 12-fold higher (p < 0.001) NAb titers and 1.7-fold higher (6.5, 95% CI 1.3 to 32, p = 0.02) response rates than BNT162b2 vaccinees despite similar disease characteristics. The absence of detectable NAbs in CLL patients was associated with reduced naïve CD4+ T cells (p = 0.03) and increased CD8+ effector memory T cells (p = 0.006). Limitations of the study were that not all participants were subjected to the same immune analyses and that pre-vaccination samples were not available.

CONCLUSIONS

CLL pathogenesis is characterized by a progressive loss of adaptive immune functions, including in most treatment-naïve patients, with preexisting memory being preserved longer than the capacity to mount responses to new antigens. In addition, higher NAb titers and response rates identify mRNA-1273 as a superior vaccine for CLL patients.

Evidence for Extensive Duplication and Subfunctionalization of FCRL6 in Armadillo (Dasypus novemcinctus)

The control of infections by the vertebrate adaptive immune system requires careful modulation to optimize defense and minimize harm to the host. The Fc receptor-like (FCRL) genes encode immunoregulatory molecules homologous to the receptors for the Fc portion of immunoglobulin (FCR). To date, nine different genes (FCRL1-6, FCRLA, FCRLB and FCRLS) have been identified in mammalian organisms. FCRL6 is located at a separate chromosomal position from the FCRL1-5 locus, has conserved synteny in mammals and is situated between the SLAMF8 and DUSP23 genes. Here, we show that this three gene block underwent repeated duplication in Dasypus novemcinctus (nine-banded armadillo) resulting in six FCRL6 copies, of which five appear functional. Among 21 mammalian genomes analyzed, this expansion was unique to D. novemcinctus. Ig-like domains that derive from the five clustered FCRL6 functional gene copies show high structural conservation and sequence identity. However, the presence of multiple non-synonymous amino acid changes that would diversify individual receptor function has led to the hypothesis that FCRL6 endured subfunctionalization during evolution in D. novemcinctus. Interestingly, D. novemcinctus is noteworthy for its natural resistance to the Mycobacterium leprae pathogen that causes leprosy. Because FCRL6 is chiefly expressed by cytotoxic T and NK cells, which are important in cellular defense responses against M. leprae, we speculate that FCRL6 subfunctionalization could be relevant for the adaptation of D. novemcinctus to leprosy. These findings highlight the species-specific diversification of FCRL family members and the genetic complexity underlying evolving multigene families critical for modulating adaptive immune protection.

SARS-CoV-2 mRNA vaccination exposes progressive adaptive immune dysfunction in patients with chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) patients have lower seroconversion rates and antibody titers following SARS-CoV-2 vaccination, but the reasons for this diminished response are poorly understood. Here, we studied humoral and cellular responses in 95 CLL patients and 30 healthy controls after two BNT162b2 or mRNA-2173 mRNA immunizations. We found that 42% of CLL vaccinees developed SARS-CoV-2-specific binding and neutralizing antibodies (NAbs), while 32% had no response. Interestingly, 26% were seropositive, but had no detectable NAbs, suggesting the maintenance of pre-existing endemic human coronavirus-specific antibodies that cross-react with the S2 domain of the SARS-CoV-2 spike. These individuals had more advanced disease. In treatment-naïve CLL patients, mRNA-2173 induced 12-fold higher NAb titers and 1.7-fold higher response rates than BNT162b2. These data reveal a graded loss of immune function, with pre-existing memory being preserved longer than the capacity to respond to new antigens, and identify mRNA-2173 as a superior vaccine for CLL patients.

CD4 + Cytotoxic T Lymphocytes in Cancer Immunity and Immunotherapy

CD4⁺ T cells have the ability to differentiate into relatively specialized effector subsets after exposure to innate immune signals. The remarkable plasticity of CD4⁺ T cells is required to achieve immune responses in different tissues and against various pathogens. Numerous studies have shown that CD4⁺ T cells can play direct and indispensable roles in protective immunity by killing infected or transformed cells. Although the lineage decision of commitment to the CD4⁺ or CD8⁺ cell lineage is once thought to be inflexible, the identification of antigen-experienced CD4⁺ T cells with cytotoxic activity suggests the existence of unexpected plasticity for these cells. The recognition of CD4⁺ cytotoxic T lymphocytes (CTLs) and the mechanisms driving the differentiation of this particular cell subset create opportunities to explore the roles of these effector cells in protective immunity and immune-related pathology. CD4⁺ CTLs are proven to play a protective role in antiviral immunity. Here, the latest investigations on the phenotypic and functional features of CD4⁺ CTLs and their roles in antitumor immunity and immunotherapy are briefly reviewed.

Phosphoantigen-stimulated γδ T cells suppress natural killer cell-responses to missing-self

γδ T cells stimulated by phosphoantigens (pAg) are potent effectors that secrete Th1 cytokines and kill tumor cells. Consequently, they are considered candidates for use in cancer immunotherapy. However, they have proven only moderately effective in several clinical trials. We studied the consequences of pAg-stimulated γδ T-cell interactions with Natural Killer (NK) cells and CD8+ T cells, major innate and adaptive effectors, respectively. We found that pAg-stimulated γδ T cells suppressed NK-cell responses to "missing-self" but had no effect on antigen-specific CD8+ T-cell responses. Extensive analysis of the secreted cytokines showed that pAg-stimulated γδ T cells had a pro-inflammatory profile. CMV-pp65-specific CD8+ T cells primed with pAg-stimulated γδ T cells showed little effect on responses to pp65-loaded target cells. By contrast, NK cells primed similarly with γδ T cells had impaired capacity to degranulate and produce IFNγ in response to HLA class I-deficient targets. This effect depended on BTN3A1 and required direct contact between NK cells and γδ T cells. γδ T cell-priming of NK cells also led to a downregulation of NKG2D and NKp44 on NK cells. Every NK-cell subset was affected by γδ T cell-mediated immunosuppression, but the strongest effect was on KIR+NKG2A- NK cells. We therefore report a previously unknown function for γδ T cells, as brakes of NK-cell responses to "missing-self". This provides a new perspective for optimizing the use of γδ T cells in cancer immunotherapy and for assessing their role in immune responses to pAg-producing pathogens.

Acute and Chronic Changes in Gene Expression After CMV DNAemia in Kidney Transplant Recipients

Cytomegalovirus (CMV) viremia continues to cause significant morbidity and mortality in kidney transplant patients with clinical complications including organ rejection and death. Whole blood gene expression dynamics in CMV viremic patients from onset of DNAemia through convalescence has not been well studied to date in humans. To evaluate how CMV infection impacts whole blood leukocyte gene expression over time, we evaluated a matched cohort of 62 kidney transplant recipients with and without CMV DNAemia using blood samples collected at multiple time points during the 12-month period after transplant. While transcriptomic differences were minimal at baseline between DNAemic and non-DNAemic patients, hundreds of genes were differentially expressed at the long-term timepoint, including genes enriching for pathways important for macrophages, interferon, and IL-8 signaling. Amongst patients with CMV DNAemia, the greatest amount of transcriptomic change occurred between baseline and 1-week post-DNAemia, with increase in pathways for interferon signaling and cytotoxic T cell function. Time-course gene set analysis of these differentially expressed genes revealed that most of the enriched pathways had a significant time-trend. While many pathways that were significantly down- or upregulated at 1 week returned to baseline-like levels, we noted that several pathways important in adaptive and innate cell function remained upregulated at the long-term timepoint after resolution of CMV DNAemia. Differential expression analysis and time-course gene set analysis revealed the dynamics of genes and pathways involved in the immune response to CMV DNAemia in kidney transplant patients. Understanding transcriptional changes caused by CMV DNAemia may identify the mechanism behind patient vulnerability to CMV reactivation and increased risk of rejection in transplant recipients and suggest protective strategies to counter the negative immunologic impact of CMV. These findings provide a framework to identify immune correlates for risk assessment and guiding need for extending antiviral prophylaxis.

Roles for the FCRL6 Immunoreceptor in Tumor Immunology

Members of the Fc receptor-like (FCRL1-6) gene family encode transmembrane glycoproteins that are preferentially expressed by B cells and generally repress responses via cytoplasmic tyrosine-based regulation. Given their distribution and function, there is a growing appreciation for their roles in lymphoproliferative disorders and as immunotherapeutic targets. In contrast to FCRL1-5, FCRL6 is distinctly expressed outside the B lineage by cytotoxic T and NK lymphocytes. Its restricted expression by these orchestrators of cell-mediated immunity, along with its inhibitory properties and extracellular interactions with MHCII/HLA-DR, represent a newly appreciated axis with relevance in tolerance and cancer defense. The significance of FCRL6 in this arena has been recently demonstrated by its upregulation in HLA-DR⁺ tumor samples from melanoma, breast, and lung cancer patients who relapsed following PD-1 blockade. These findings imply a potential mechanistic role for FCRL6 in adaptive evasion to immune checkpoint therapy. Here we review these new developments in the FCRL field and identify new evidence for the prognostic significance of FCRL6 in malignancies that collectively indicate its potential as a biomarker and therapeutic target.

Fc Receptor-Like 6 (FCRL6) Discloses Progenitor B Cell Heterogeneity That Correlates With Pre-BCR Dependent and Independent Pathways of Natural Antibody Selection

B-1a cells produce "natural" antibodies (Abs) to neutralize pathogens and clear neo self-antigens, but the fundamental selection mechanisms that shape their polyreactive repertoires are poorly understood. Here, we identified a B cell progenitor subset defined by Fc receptor-like 6 (FCRL6) expression, harboring innate-like defense, migration, and differentiation properties conducive for natural Ab generation. Compared to FCRL6- pro B cells, the repressed mitotic, DNA damage repair, and signaling activity of FCRL6+ progenitors, yielded VH repertoires with biased distal Ighv segment accessibility, constrained diversity, and hydrophobic and charged CDR-H3 sequences. Beyond nascent autoreactivity, VH11 productivity, which predominates phosphatidylcholine-specific B-1a B cell receptors (BCRs), was higher for FCRL6+ cells as was pre-BCR formation, which was required for Myc induction and VH11, but not VH12, B-1a development. Thus, FCRL6 revealed unexpected heterogeneity in the developmental origins, regulation, and selection of natural Abs at the pre-BCR checkpoint with implications for autoimmunity and lymphoproliferative disorders.

Fc Receptor-Like 1 as a Promising Target for Immunotherapeutic Interventions of B-Cell-Related Disorders

BACKGROUND

Human B-cell responses are regulated through synergy between a collection of activation and inhibitory receptors. Fc receptor-like (FCRL) molecules have recently been identified as co-receptors that are preferentially expressed in human B-cells, which may also play an important role in the regulation of human B-cell responses. FCRL1 is a member of the FCRL family molecules with 2 immunoreceptor tyrosine-based activation motifs (ITAMs) in its cytoplasmic tail. This study aimed to investigate the regulatory roles of FCRL1 in human B-cell responses.

MATERIALS AND METHODS

The regulatory potential of FCRL1 in human B-cell through knockdown of FCRL1 expression in the Ramos and Daudi Burkitt lymphoma (BL) cell lines by using the retroviral-based short hairpin ribonucleic acid (shRNA) delivery method. The functional consequences of FCRL1 knockdown were assessed by measuring the proliferation, apoptosis, and the expression levels of Bcl-2, Bid, and Bax genes as well as phosphoinositide-3 kinase/-serine-threonine kinase AKT (PI3K/p-AKT) pathway in the BL cells, using the quantitative real-time polymerase chain reaction (PCR) and flow cytometry analysis. The NF-κB activity was also measured by the enzyme-linked immunosorbent assay (ELISA).

RESULTS

FCRL1 knockdown significantly decreased cell proliferation and increased apoptotic cell death in the BL cells. There was a significant reduction in the extent of the Bcl-2 gene expression in the treated BL cells compared with control cells. On the contrary, FCRL1 knockdown increased the expression levels of Bid and Bax genes in the treated BL cells when compared with control cells. In addition, the extent of the PI3K/p-AKT expression and phosphorylated-p65 NF-κB activity was significantly decreased in the treated BL cells compared with control cells.

CONCLUSIONS

These results suggest that FCRL1 can play a key role in the activation of human B-cell responses and has the potential to serve as a target for immunotherapy of FCRL1 positive B-cell-related disorders.

Prognostic significance of Fc receptor-like 1 in patients with chronic lymphocytic leukemia, hairy cell leukemia, and various B-cell non-Hodgkin's lymphoma

Fc receptor-like 1 (FCRL1) positively regulates B-cell responses and may involve in the pathogenesis of B-cell malignancies. This study examined the expression pattern of FCRL1 in B-cell non-Hodgkin's lymphoma patients using real-time PCR and flow cytometry. The results revealed higher levels of FCRL1 expression in diffuse large B-cell lymphoma, hairy cell leukemia, and Burkitt lymphoma patients compared with control groups. There was a significant reduction in the levels of FCRL1 expression in chronic lymphocytic leukemia and mantle cell lymphoma patients compared with healthy individuals. These findings suggest FCRL1 as an excellent marker for the prognosis or immunotherapy of B-cell malignancies.

Primary EBV Infection Induces an Acute Wave of Activated Antigen-Specific Cytotoxic CD4+ T Cells

Primary EBV infection drives highly cytotoxic virus-specific CD4⁺ T cell responses.

EBV-specific memory CD4⁺ T cells are polyfunctional but lack cytotoxic activity.

Acute EBV-specific CD4-CTLs differ transcriptionally from classical memory CD4-CTLs.

CD4⁺ T cells are essential for immune protection against viruses, yet their multiple roles remain ill-defined at the single-cell level in humans. Using HLA class II tetramers, we studied the functional properties and clonotypic architecture of EBV-specific CD4⁺ T cells in patients with infectious mononucleosis, a symptomatic manifestation of primary EBV infection, and in long-term healthy carriers of EBV. We found that primary infection elicited oligoclonal expansions of T_H1-like EBV-specific CD4⁺ T cells armed with cytotoxic proteins that responded immediately ex vivo to challenge with EBV-infected B cells. Importantly, these acutely generated cytotoxic CD4⁺ T cells were highly activated and transcriptionally distinct from classically described cytotoxic CD4⁺ memory T cells that accumulate during other persistent viral infections, including CMV and HIV. In contrast, EBV-specific memory CD4⁺ T cells displayed increased cytokine polyfunctionality but lacked cytotoxic activity. These findings suggested an important effector role for acutely generated cytotoxic CD4⁺ T cells that could potentially be harnessed to improve the efficacy of vaccines against EBV.

Tumor-specific MHC-II expression drives a unique pattern of resistance to immunotherapy via LAG-3/FCRL6 engagement

Immunotherapies targeting the PD-1 pathway produce durable responses in many cancers, but the tumor-intrinsic factors governing response and resistance are largely unknown. MHC-II expression on tumor cells can predict response to anti-PD-1 therapy. We therefore sought to determine how MHC-II expression by tumor cells promotes PD-1 dependency. Using transcriptional profiling of anti-PD-1-treated patients, we identified unique patterns of immune activation in MHC-II+ tumors. In patients and preclinical models, MHC-II+ tumors recruited CD4+ T cells and developed dependency on PD-1 as well as Lag-3 (an MHC-II inhibitory receptor), which was upregulated in MHC-II+ tumors at acquired resistance to anti-PD-1. Finally, we identify enhanced expression of FCRL6, another MHC-II receptor expressed on NK and T cells, in the microenvironment of MHC-II+ tumors. We ascribe this to what we believe to be a novel inhibitory function of FCRL6 engagement, identifying it as an immunotherapy target. These data suggest a MHC-II-mediated context-dependent mechanism of adaptive resistance to PD-1-targeting immunotherapy.

Tumor-specific MHC-II expression drives a unique pattern of resistance to immunotherapy via LAG-3/FCRL6 engagement

Immunotherapies targeting the PD-1 pathway produce durable responses in many cancers, but the tumor-intrinsic factors governing response and resistance are largely unknown. MHC-II expression on tumor cells can predict response to anti-PD-1 therapy. We therefore sought to determine how MHC-II expression by tumor cells promotes PD-1 dependency. Using transcriptional profiling of anti-PD-1-treated patients, we identified unique patterns of immune activation in MHC-II+ tumors. In patients and preclinical models, MHC-II+ tumors recruited CD4+ T cells and developed dependency on PD-1 as well as Lag-3 (an MHC-II inhibitory receptor), which was upregulated in MHC-II+ tumors at acquired resistance to anti-PD-1. Finally, we identify enhanced expression of FCRL6, another MHC-II receptor expressed on NK and T cells, in the microenvironment of MHC-II+ tumors. We ascribe this to what we believe to be a novel inhibitory function of FCRL6 engagement, identifying it as an immunotherapy target. These data suggest a MHC-II-mediated context-dependent mechanism of adaptive resistance to PD-1-targeting immunotherapy.

Update on Fc receptor-like (FCRL) family: new immunoregulatory players in health and diseases

INTRODUCTION

Fc receptor-like (FCRL) molecules, as recently identified members of the immunoglobulin superfamily (IgSF), are preferentially expressed by B-cells. They have variable number of extracellular immunoglobulin-like domains and cytoplasmic activating ITAMs and/or inhibitory ITIMs. FCRL1-5 are dominantly expressed in different stages of B-cells development. But, FCRL6 is preferentially expressed in different subsets of T-cells and NK cells. FCRL1-5 could regulate different features of B-cell evolution such as development, differentiation, activation, antibody secretion and isotype switching. Areas covered: Improved understanding of FCRL expression may grant B-cells and finally its signaling pathways, alone or in cooperation with other signaling molecules, as interesting new targets for diagnostic, monitoring and immunotherapeutic modalities; although further investigations remain to be defined. Recent investigations on different family members of FCRL proteins have substantiated their differential expression on different tissues, malignancies, immune related disease and infectious diseases. Expert opinion: FCRLs restricted expressions in normal B-cells and T-cell subsets accompanied with their overexpression in B-cell malignancies introduce them as logical candidates for the development of antibody- and cell-based immunotherapy approaches in B-cell malignancies, immune-mediated and infectious diseases. FCRLs would be applied as attractive and specific targets for immunodiagnostic approaches, clinical prognosis as well as disease monitoring of relevant patients.

Whole transcriptome profiling reveals major cell types in the cellular immune response against acute and chronic active Epstein-Barr virus infection

Epstein-Barr virus (EBV) is a common human pathogen that infects over 95% of the population worldwide. In the present study, the whole transcriptome microarray data were generated from peripheral blood mononuclear cells from Chinese children with acute infectious mononucleosis (AIM) and chronic active EBV infection (CAEBV) that were also compared with a publicly available microarray dataset from a study of American college students with AIM. Our study characterized for the first time a broad spectrum of molecular signatures in AIM and CAEBV. The key findings from the transcriptome profiling were validated with qPCR and flow cytometry assays. The most important finding in our study is the discovery of predominant γδ TCR expression and γδ T cell expansion in AIM. This finding, in combination with the striking up-regulation of CD3, CD8 and CD94, suggests that CD8+ T cells and CD94+ NK cells may play a major role in AIM. Moreover, the unique up-regulation of CD64A/B and its significant correlation with the monocyte marker CD14 was observed in CAEBV and that implies an important role of monocytes in CAEBV. In conclusion, our study reveals major cell types (particularly γδ T cells) in the host cellular immune response against AIM and CAEBV.

Cytotoxic CD4 T Cells-Friend or Foe during Viral Infection?

CD4 T cells with cytotoxic function were once thought to be an artifact due to long-term in vitro cultures but have in more recent years become accepted and reported in the literature in response to a number of viral infections. In this review, we focus on cytotoxic CD4 T cells in the context of human viral infections and in some infections that affect mice and non-human primates. We examine the effector mechanisms used by cytotoxic CD4 cells, the phenotypes that describe this population, and the transcription factors and pathways that lead to their induction following infection. We further consider the cells that are the predominant targets of this effector subset and describe the viral infections in which CD4 cytotoxic T lymphocytes have been shown to play a protective or pathologic role. Cytotoxic CD4 T cells are detected in the circulation at much higher levels than previously realized and are now recognized to have an important role in the immune response to viral infections.

Fc Receptor-Like Proteins in Pathophysiology of B-cell Disorder

Members of the family of Fc receptor-like (FcRL) proteins, homologous to FcγRI, have been identified by multiple research groups. Consequently, they have been described using multiple nomenclatures including Fc receptor homologs (FcRH), immunoglobulin superfamily receptor translocation-associated genes (IRTA), immunoglobulin-Fc-gp42-related genes (IFGP), Src homology 2 domain-containing phosphatase anchor proteins (SPAP), and B cell cross-linked by anti-immunoglobulin M-activating sequences (BXMAS). They are now referred to under a unified nomenclature as FCRL. Eight different human FCRL genes have been identified, all of which appear to be related to the genes of the immunoglobulin superfamily (IgSF) of cellular adhesion molecules. These type 1 transmembrane glycoproteins are composed of different combinations of 5 types of immunoglobulin-like domains, with each protein consisting of 3 to 9 domains, and no individual domain type conserved throughout all of the FCRL proteins. Ligands for the majority of the FCRLs remain unknown. In general, FCRL expression is restricted to lymphocytes and is primarily expressed in B-lymphocytes, supporting FCRL’s involvement in a variety of immune disorders. Most FCRLs functionally repress B-cell activation; however, they might have dual roles in lymphocyte functions as these proteins often possess immunoreceptor tyrosine activation (ITAM) and inhibitory (ITIM) motif elements. The biological functions of these newly recognized FCRL proteins are just beginning to emerge, and might provide the insight necessary for understanding pathophysiology of lymphocyte disorders and treating different immune diseases.

A Leukocyte Immune-Type Receptor Subset Is a Marker of Antiviral Cytotoxic Cells in Channel Catfish, Ictalurus punctatus

Channel catfish, Ictalurus punctatus, leukocyte immune type receptors (LITRs) represent a multigene family that encodes Ig superfamily proteins that mediate activating or inhibitory signaling. In this study, we demonstrate the use of mAb CC41 to monitor viral cytotoxic responses in catfish and determine that CC41 binds to a subset of LITRs on the surface of catfish clonal CTLs. Homozygous gynogenetic catfish were immunized with channel catfish virus (CCV)-infected MHC-matched clonal T cells (G14D-CCV), and PBL were collected at various times after immunization for flow cytometric analyses. The percentage of CC41(+) cells was significantly increased 5 d after primary immunization with G14D-CCV and at 3 d after a booster immunization as compared with control fish only injected with G14D. Moreover, CC41(+) cells magnetically isolated from the PBL specifically killed CCV-infected targets as measured by (51)Cr release assays and expressed messages for CD3γδ, perforin, and at least one of the CD4-like receptors as analyzed by RNA flow cytometry. When MLC effector cells derived from a G14D-CCV-immunized fish were preincubated with CC41 mAb, killing of G14D-CCV targets was reduced by ∼40%, suggesting that at least some LITRs have a role in target cell recognition and/or cytotoxicity. The availability of a LITR-specific mAb has allowed, to our knowledge for the first time, functional characterization of LITRs in an autologous system. In addition, the identification of an LITR subset as a cytotoxic cell marker will allow for more effective monitoring of catfish immune responses to pathogens.

Emerging roles for the FCRL family members in lymphocyte biology and disease

Members of the extended Fc receptor-like (FCRL) family in humans and mice are preferentially expressed by B cells and possess tyrosine-based immunoregulatory function. Although the majority of these proteins repress B cell receptor-mediated activation, there is an emerging evidence for their bifunctionality and capacity to counter-regulate adaptive and innate signaling pathways. In light of these findings, the recent discovery of ligands for several of these molecules has begun to reveal exciting potential for them in normal lymphocyte biology and is launching a new phase of FCRL investigation. Importantly, these fundamental developments are also setting the stage for defining their altered roles in the pathogenesis of a growing number of immune-mediated diseases. Here we review recent advances in the FCRL field and highlight the significance of these intriguing receptors in normal and perturbed immunobiology.

Expanded CD8+ T cells of murine and human CLL are driven into a senescent KLRG1+ effector memory phenotype

Altered numbers and functions of T cells have previously been demonstrated in chronic lymphocytic leukemia (CLL) patients. However, dynamics and specific T-cell subset alterations have not been studied in great detail. Therefore, we studied CLL blood lymphocyte subsets of individual patients in a longitudinal manner. Dynamic expansions of blood CD4 + and CD8 + T-cell numbers were consistently associated with a progressively increasing CLL leukemic compartment. Interestingly, the T-cell subset expansion over time was more pronounced in CD38 + CLL. Additionally, we performed gene expression profiling of CD3 + T cells of CLL patients and normal donors. Using gene set enrichment analysis, we found significant enrichment of genes with higher expression in CLL T cells within CD8+ effector memory and terminal effector T-cell gene signatures. In agreement with these data, we observed a marked expansion of phenotypic CD8 + effector memory T cells in CLL by flow cytometry. Moreover, we observed that increments of CD8 + effector memory T cells in human CLL and also mouse CLL (Eμ-TCL1 model) were due to an expansion of the inhibitory killer cell lectin-like receptor G1 (KLRG1) expressing cellular subset. Furthermore, higher plasma levels of the natural KLRG1 ligand E-cadherin were detected in CLL patients compared to normal donor controls. The predominance of KLRG1+ expression within CD8+ T cells in conjunction with increased systemic soluble E-cadherin might significantly contribute to CLL immune dysfunction and might additionally represent an important component of the CLL microenvironment.

FCRL3 promotes TLR9-induced B-cell activation and suppresses plasma cell differentiation

Fc receptor-like (FCRL) molecules are preferentially expressed by B lymphocytes and possess tyrosine-based immunoregulatory function. Although they generally inhibit B-cell receptor signaling, their influence on other activation pathways remains largely unexplored. In humans, FCRL3 encodes a type I transmembrane protein harboring both cytoplasmic ITAM and ITIM elements that can repress B-cell receptor activation. Despite this inhibitory property, mounting associations for FCRL3 with autoimmune and lympho-proliferative disorders imply a role for it in promoting B-cell pathogenesis. Here, we explore the influence of FCRL3 on B-cell responses to innate TLR9 stimulation. A detailed survey of blood B-cell populations found that FCRL3 expression increased as a function of differentiation and was higher among memory subsets with innate-like features. FCRL3 ligation augmented CpG oligodeoxynucleotide TLR9-mediated B-cell proliferation, activation, and survival, but surprisingly, abrogated plasma cell differentiation and antibody production. Although FCRL3 amplified the NF-κB and mitogen-activated protein kinase signaling cascades, it halted CpG triggered BLIMP1 induction in an ERK-dependent fashion. These findings indicate that FCRL3 differentially modulates innate signaling in B cells and provide new insight into the potential of this disease-associated receptor to counter-regulate adaptive and innate immunity.

Cutting edge: human FcRL4 and FcRL5 are receptors for IgA and IgG

Fc receptor-like (FcRL) proteins are a family of cellular receptors homologous to FcγRI and are predominantly expressed by B cells. They function to costimulate or inhibit BCR signaling through consensus ITAMs and ITIMs; however, the extracellular ligands of these receptors remain unknown or controversial. In this study, we tested the ability of human FcRL proteins to bind Igs and found FcRL4 and FcRL5 to be bona fide Fc receptors. In cellular binding assays, FcRL4 bound efficiently to IgA and FcRL5 binds all IgG isotypes with varied efficiency. Additionally, we generated mAbs capable of specifically blocking these interactions. Given their expression on activated B cells and potential for inhibitory signaling, FcRL4 and FcRL5 are likely to be important for immune complex-dependent human B cell regulation, and they represent novel therapeutic targets for receptor blockade therapies.

Expansion of a CD8(+)PD-1(+) replicative senescence phenotype in early stage CLL patients is associated with inverted CD4:CD8 ratios and disease progression

PURPOSE

Patients with chronic lymphocytic leukemia (CLL) display immune deficiency that is most obvious in advanced stage disease. Here we investigated whether this immune dysfunction plays a pathologic role in the progression of early stage disease patients.

EXPERIMENTAL DESIGN

We carried out eight-color immunophenotyping analysis in a cohort of 110 untreated early stage CLL patients and 22 age-matched healthy donors and correlated our findings with clinical outcome data.

RESULTS

We found a significant reduction in naive CD4(+) and CD8(+) T cells in CLL patients. Only the CD4(+) subset showed significantly increased effector memory cells (T(EM) and T(EMRA)) in the whole cohort (P = 0.004 and P = 0.04, respectively). However, patients with inverted CD4:CD8 ratios (52 of 110) showed preferential expansion of the CD8 compartment, with a skewing of CD8(+) T(EMRA) (P = 0.03) coupled with increased percentage of CD57(+)CD28(-)CD27(-) T cells (P = 0.008) and PD-1 positivity (P = 0.027), consistent with a replicative senescence phenotype. Furthermore, inverted CD4:CD8 ratios were associated with shorter lymphocyte doubling time (P = 0.03), shorter time to first treatment (P = 0.03), and reduced progression-free survival (P = 0.005).

CONCLUSIONS

Our data show that the emergence of CD8(+)PD-1(+) replicative senescence phenotype in early stage CLL patients is associated with more aggressive clinical disease. Importantly, these findings were independent of tumor cell prognostic markers and could not be accounted for by patient age, changes in regulatory T-cell frequency, or cytomegalovirus serostatus (n = 217).

FCRLA is a resident endoplasmic reticulum protein that associates with intracellular Igs, IgM, IgG and IgA

Fc receptor-like A (FCRLA) is an unusual member of the extended Fc receptor family. FCRLA has homology to receptors for the Fc portion of Ig (FCR) and to other FCRL proteins. However, unlike these other family representatives, which are typically transmembrane receptors with extracellular ligand-binding domains, FCRLA has no predicted transmembrane domain or N-linked glycosylation sites and is an intracellular protein. We show by confocal microscopy and biochemical assays that FCRLA is a soluble resident endoplasmic reticulum (ER) protein, but it does not possess the amino acid sequence KDEL as an ER retention motif in its C-terminus. Using a series of deletion mutants, we found that its ER retention is most likely mediated by the amino terminal partial Ig-like domain. We have identified ER-localized Ig as the FCRLA ligand. FCRLA is unique among the large family of Fc receptors, in that it is capable of associating with multiple Ig isotypes, IgM, IgG and IgA. Among hemopoietic cells, FCRLA expression is restricted to the B lineage and is most abundant in germinal center B lymphocytes. The studies reported here demonstrate that FCRLA is more broadly expressed among human B lineage cells than originally reported; it is found at significant levels in resting blood B cells and at varying levels in all B-cell subsets in tonsil.

Cutting edge: FcR-like 6 is an MHC class II receptor

Receptors for the Fc portion of Ig have been extensively characterized and are known to regulate humoral responses, but members of the closely related FcR-like (FCRL) family have not been found to bind Ig, and to date, no ligand has been identified for any FCRL. Using a cell-based GFP reporter system and a recombinant Fc chimeric protein, we show that human FCRL6, a receptor selectively expressed by cytotoxic T and NK cells, directly binds HLA-DR, an MHC class II molecule. Given the similarity among constant regions of Ig and MHC molecules, these findings suggest that representatives of the FcR and FCRL multigene families may have independently evolved to engage two ancestral elements fundamental to adaptive immunity. This discovery may offer new insight into the interaction between cytotoxic lymphocytes and APCs and may have important implications for better understanding HLA disease susceptibility and pathogenesis.

Immunoregulatory roles for fc receptor-like molecules

Fc receptor-like (FCRL) molecules comprise a family of imunoregulatory transmembrane proteins that are preferentially, but not exclusively expressed on B lineage cells. A strong regulatory potential on B cell activation has been characterized for the different FCRL proteins, but their biological roles are just beginning to be elucidated. We review recent advances in the understanding of FCRL1-6 expression and function, and indicate their potential roles in the pathogenesis of immunodeficiencies, lymphoid malignancies and autoimmune diseases.

Fc receptor-like 3 protein expressed on IL-2 nonresponsive subset of human regulatory T cells

Fc receptor-like 3 (FCRL3) is a cell surface protein homologous to Fc receptors. The FCRL3 gene is present in humans but not in mice. We found that FCRL3 protein is expressed on 40% of human naturally occurring CD4(+) regulatory T (nTreg) cells (CD4(+)CD25(+)CD127(low)). Sorted nTreg cells with the surface phenotype FCRL3(+) and FCRL3(-) were both hypoproliferative to TCR stimulation and both suppressive on proliferation of conventional T cells (CD4(+)CD25(-)) in vitro. They both expressed forkhead box p3 (Foxp3) protein, the intracellular regulatory T cell marker. However, in contrast to FCRL3(-) nTreg cells, FCRL3(+) nTreg cells were not stimulated to proliferate by the addition of exogenous IL-2. In addition, Foxp3(+) cells induced from conventional T cells by TGF-beta treatment did not exhibit FCRL3 expression. These results suggest that the FCRL3(+) subset of human nTreg cells identified in this study arise in vivo and Foxp3 expression alone is not sufficient to induce FCRL3 expression. FCRL3 may be involved in human-specific mechanisms to control the generation of nTreg cells.