ARHGEF1 deficiency reveals Gα13-associated GPCRs are critical regulators of human lymphocyte function Divij Mathew, … , Kimberly N. Kremer, Raul M. Torres J Clin Invest. 2019;129(3):965-968. https://doi.org/10.1172/JCI125893. Commentary Primary antibody deficiencies are the most common immunodeficiencies in humans; however, identification of the underlying genetic and biochemical basis for these diseases is often difficult, given that these deficiencies typically involve complex genetic etiologies. In this issue of the JCI, Bouafia et al. performed whole-exome sequencing on a pair of siblings with primary antibody deficiencies and identified genetic mutations that result in a deficiency of ARHGEF1, a hematopoietic intracellular signaling molecule that transmits signals from GPCRs. ARHGEF1-deficient lymphocytes from the affected siblings exhibited important functional deficits that indicate that loss of ARHGEF1 accounts for the observed primary antibody deficiency, which manifests in an inability to mount antibody responses to vaccines and pathogens. Thus, this report demonstrates an important role for ARHGEF1 in GPCR signal transduction required for appropriate adaptive immune responses in humans. Find the latest version: https://jci.me/125893/pdf The Journal of Clinical Investigation COMMENTARY ARHGEF1 deficiency reveals Gα13-associated GPCRs are critical regulators of human lymphocyte function Divij Mathew, Kimberly N. Kremer, and Raul M. Torres Department of Immunology and Microbiology, University of Colorado School of Medicine, Aurora, Colorado, USA. both Gα12 and Gα13, only the association Primary antibody deficiencies are the most common immunodeficiencies in with active Gα13 stimulates its RhoA GEF humans; however, identification of the underlying genetic and biochemical activity (10). Thus, based on these bio- basis for these diseases is often difficult, given that these deficiencies chemically defined activities, ARHGEF1 typically involve complex genetic etiologies. In this issue of the JCI, Bouafia functions to terminate GPCR signaling et al. performed whole-exome sequencing on a pair of siblings with primary from Gα13-associated receptors and sub- antibody deficiencies and identified genetic mutations that result in a sequently activate RhoA, an important deficiency of ARHGEF1, a hematopoietic intracellular signaling molecule that regulator of the actin cytoskeleton with pleiotropic functions. transmits signals from GPCRs. ARHGEF1-deficient lymphocytes from the ARHGEF1 is predominantly expressed affected siblings exhibited important functional deficits that indicate that in cells of hematopoietic origin, and as loss of ARHGEF1 accounts for the observed primary antibody deficiency, GPCRs are well characterized as regulat- which manifests in an inability to mount antibody responses to vaccines and ing diverse activities in immune cells, such pathogens. Thus, this report demonstrates an important role for ARHGEF1 as trafficking, differentiation, survival, in GPCR signal transduction required for appropriate adaptive immune and proliferation, several Arhgef1–/– mouse responses in humans. mutants were independently generated (11–13) to investigate how Gα12/13-asso- ciated GPCRs use ARHGEF1 to regulate these functions. The aggregate results Introduction antibody responses. These analyses led to from analyses of ARHGEF1 mouse Primary antibody deficiencies (PADs) the identification of two distinct ARHGEF1 mutants revealed an important role for include a broad spectrum of disorders that mutant alleles that, together as compound ARHGEF1 in the development of mature B range from the absence of B lymphocytes heterozygous mutations, resulted in an cell populations (e.g., marginal zone [MZ] and serum Ig to normal serum Ig isotypes ARHGEF1 deficiency. B cells) and B lymphocyte migration that, and concentrations, but aberrant anti- when disrupted, ultimately resulted in body responses (1, 2). The identification of ARHGEF1 markedly impaired antigen-specific anti- genetic mutations that underlie and lead ARHGEF1 was first identified over two body responses to both T cell–dependent to PADs, and in particular X-linked agam- decades ago (6, 7) as an intracellular sig- and T cell–independent antigens (11, 12). maglobulinemias, has benefited from the naling molecule with two biochemically Although the precise Gα12/13-associated analyses of natural and engineered genet- defined functions (Figure 1). The first GPCR or GPCRs responsible for the devel- ic mutations in mouse models that lead to activity is contained within a regulator opment and function of MZ B cells are not impaired B cell development (3, 4). How- of G-protein signaling (RGS) domain yet fully identified, the results from these ever, in the current era of –omics (genom- that functions to accelerate the inherent mouse models indicate that a Gα13-asso- ics, proteomics, lipidomics, and metabalo- GTPase activity of Gα heterotrimeric ciated GPCR/ARHGEF1 signaling path- mics), it is now possible to identify genetic G-protein subunits (8). ARHGEF1 RGS way is important for MZ B cell–mediated lesions that underlie symptoms in affected activity is specific for Gα12 and Gα13 sub- humoral immunity. Further studies are patients and subsequently characterize units, and thus ARHGEF1 attenuates sig- necessary to identify which GPCRs con- the manifestation of these mutations on naling from Gα12/13-associated GPCRs tribute to the development of humoral lymphocyte function. In this issue, Boua- (9). The second activity is contained immunity and whether these GPCRs func- fia et al. (5) report on their performance within a tandem Dbl- and pleckstrin- tion to activate specific transcriptional and analysis of whole-exome sequenc- homology (DH-PH) domain that har- programs, position MZ B cells in distinct ing of siblings presenting with recurrent bors GEF activity specific for the small anatomical locations, or promote specific respiratory tract infection, bronchiectasis, molecular weight RhoA GTPase (10). cell-cell interactions that are important for and an inability to mount antigen-specific Although ARHGEF1 RGS activity acts on appropriate differentiation. Related Article: p. 1047 Clinical manifestations of an ARHGEF1 deficiency Conflict of interest: The authors have declared that no conflict of interest exists. Bouafia et al. (5) focus on a set of ARH- Reference information: J Clin Invest. 2019;129(3):965–968. https://doi.org/10.1172/JCI125893. GEF1-deficient siblings who presented jci.org Volume 129 Number 3 March 2019 965 COMMENTARY The Journal of Clinical Investigation Figure 1. ARHGEF1 is an intracellular signaling effector that activates RhoA in response to Gα12/13-associated GPCR signaling. (A) ARH- GEF1 RGS activity terminates Gα12/13-associ- ated GPCR signaling and stimulates ARHGEF1 RhoA GEF activity. Active GTP-bound RhoA controls actin cytoskeleton and is required for appropriate integrin adhesion and migration. (B) ARHGEF1-deficient lymphocytes would not effi- ciently terminate GPCR signaling nor activate RhoA, resulting in impaired Gα12/13-associated GPCR signaling through S1P, chemokine, and LPA GPCRs. As a result, ARHGEF1-deficient lymphocytes display impaired adhesion and trafficking, ultimately leading to aberrant localization in SLOs and germinal centers and defective antigen-specific antibody responses. early in childhood with recurrent upper ARHGEF1 signaling functions cits are reminiscent of previous analyses and lower respiratory tract infections, important for humoral of engineered mouse mutants, including depressed serum concentrations of IgG, immunity Arhgef1–/– and S1pr2–/– mice (11–13, 15), and and, in one sibling, reduced IgA and IgE The presence of B lymphocytes was mark- are consistent with characterized Gα13- levels. Notably, neither sibling harbored edly diminished in the peripheral blood of associated GPCR signaling pathways, measurable antibody titers against pro- both ARHGEF1-deficient siblings. Conse- such as CXCR4 and LPARs/S1PRs (16– tein and viral vaccines or Streptococcus quently, to document signaling abnormal- 18). Notably, CXCR4, like all chemokine pneumonia, indicating defective antigen- ities in ARHGEF1-deficienct lymphocytes receptors, signals chemotactic migration specific antibody responses to both pro- isolated from affected siblings, Bouafia et via pertussis toxin–sensitive Gαi-associ- tein antigens and pneumococcal polysac- al. (5) relied largely on primary T cells and ated heterotrimeric proteins. However, charides. Work from rodent models has T cell blasts in most of their in vitro exper- CXCR4 expressed by certain cell types has established that protein antigens elicit iments; however, transformed mutant been shown to use Gα13-associated signal- antibody responses from B cells that are B cells had analogous results in some ing to Rho to contribute to cell migration, dependent on T cell help (TD antibody experiments. More precisely, the authors and in T cells, a CXCR4/Gα13/RhoA axis responses), whereas antibody respons- treated ARHGEF1-deficient lymphocytes has been demonstrated to be important es against capsular polysaccharides can with agonist ligands for Gα13-associated in recycling CXCR4 back to the cell sur- be elicited independently of T cells (TI GPCRs, including sphingosine-1-phos- face after ligation with CXCL12 (16–18). antibody responses) (14). Indeed, con- phate (S1P), lysophosphatidic acid (LPA), Thus, the immunological deficits of these sistent with the observed defective anti- thromboxane, and CXCL12, and retrovi- patients