The Interaction of Anti-DNA Antibodies with DNA: Evidence for Unconventional Binding Mechanisms.

Antibodies to DNA (anti-DNA) are the serological hallmark of systemic lupus erythematosus, a prototypic autoimmune disease. These antibodies bind to conserved sites on single-stranded and double-stranded DNA and display variable region somatic mutations consistent with antigen selection. Nevertheless, the interaction of anti-DNA with DNA has unconventional features. Anti-DNA antibodies bind by a mechanism called monogamous bivalency, in which stable interaction requires contact of both Fab sites with determinants on the same extended DNA molecule; the size of this DNA can be hundreds to thousands of bases, especially in solid phase assays. This binding also requires the presence of the Fc portion of IgG, a binding mechanism known as Fc-dependent monogamous bivalency. As shown by the effects of ionic strength in association and dissociation assays, anti-DNA binding is primarily electrostatic. Like anti-DNA autoantibodies, anti-DNA antibodies that bind specifically to non-conserved sites on bacterial DNA, a type of anti-DNA found in otherwise healthy individuals, also interact by monogamous bivalency. The unconventional features of anti-DNA antibodies may reflect the highly charged and polymeric nature of DNA and the need for molecular rearrangements to facilitate monogamous bivalency; the Fc portion contributes to binding in an as yet unknown way.

An Atypical Myelomonocytic Cell Infiltrate: Use of Next-Generation Sequencing to Diagnose Indeterminate Cell Histiocytosis.

ABSTRACT: Indeterminant cell histiocytosis (ICH) is a rare lymphoproliferative disorder that demonstrates features of Langerhans and non-Langerhans cell histiocytoses and diagnosis can be challenging. We present a case of a 62 year old woman with a generalized eruption of erythematous papules on the face, trunk and extremities. Skin biopsies demonstrated a dermal mononuclear cell infiltrate with monocytic (CD4, CD33), histiocytic (CD68, CD163), and dendritic cell (CD1a) immunophenotype but negative for Langerhans' cell marker (CD207). The differential diagnosis included leukemia cutis and ICH, and further workup revealed a normal bone marrow biopsy. To confirm the diagnosis of ICH, next generation sequencing with ETV3-NCOA2 gene fusion was performed and was positive. The patient's condition improved with methotrexate and narrow band UVB phototherapy. Our case adds to the existing literature supporting the use of next-generation sequencing to test for ETV3-NCOA2 gene fusion in suspected cases of ICH.

Ruxolitinib for the treatment of steroid refractory pediatric chronic graft-versus-host disease.

Chronic graft-versus-host disease (cGVHD) of the skin is a serious cause of long-term morbidity and mortality among patients who receive hematopoietic stem cell transplants. Systemic corticosteroids remain first-line treatment for cutaneous cGVHD; however, there is currently no consensus on second-line therapy for steroid-refractory disease. We herein present a case of a pediatric patient with severe cGVHD of the skin, nonresponsive to corticosteroids, who was successfully treated with a prolonged course of ruxolitinib with minimal side effects.

The Binding Mechanisms of Antibodies to DNA from Healthy Subjects and Patients with Systemic Lupus Erythematosus: The Role of Monogamous Bivalency and Fc Dependence.

Abs to DNA (anti-DNA) are a unique population of Abs that bind structural determinants on the DNA molecule. In systemic lupus erythematosus (SLE), anti-DNA Abs bind to conserved antigenic determinants, with the phosphodiester backbone being the most likely. In contrast, otherwise healthy subjects (HS) express anti-DNA that bind selectively to nonconserved sites on certain bacterial and viral DNA. As shown previously, SLE anti-DNA bind by a mechanism termed Fc-dependent monogamous bivalency. In this mechanism, both Fab sites interact with determinants on the same extended DNA molecule, reflecting the low affinity of each Fab site; the requirement for the Fc region suggests some contribution of the C region to increase avidity. In this study, we investigated whether anti-DNA from HS also bind to bacterial DNA by Fc-dependent monogamous bivalency. For this purpose, we compared the activity of intact IgG with Fab and F(ab')2 fragments prepared from the plasmas of SLE patients and HS using ELISAs with DNA from calf thymus or Micrococcus luteus These studies showed that Fab fragments from all plasmas tested, both SLE and HS, failed to bind significantly to DNA compared with intact IgG. By contrast, some, but not all, F(ab')2 preparations from both SLE patients and HS showed binding to M. luteus DNA; F(ab')2 fragments from SLE plasmas, however, did not bind significantly to calf thymus DNA. Together, these findings suggest that although anti-DNA Abs, whether from SLE or HS, bind by monogamous bivalency, binding to bacterial DNA does not require the Fc region.

Parathyroid Hormone Senses Extracellular Calcium To Modulate Endocrine Signaling upon Binding to the Family B GPCR Parathyroid Hormone 1 Receptor.

Parathyroid hormone (PTH) binds to a family B G protein coupled receptor, parathyroid hormone 1 receptor (PTH1R). One of its functions is to regulate Ca2+ homeostasis in bone remodeling, during which Ca2+ can reach up to 40 mM. A truncated version of PTH, PTH(1-34), can fully activate PTH1R and has been used for osteoporosis treatments. Here, we used fluorescence anisotropy to examine the binding of PTH(1-34) to PTH1R purified in nanodiscs (PTH1R-ND) and found that the affinity increases 5-fold in the presence of 15 mM Ca2+. However, PTHrP(1-36), another truncated endogenous agonist for PTH1R, does not show this Ca2+ effect. Mutations of Glu19 and Glu22 in PTH(1-34) that are not conserved in PTHrP(1-36) largely abolished the Ca2+ effect. The results support that PTH(1-34) not only activates PTH1R but also uniquely senses Ca2+. This dual function of a peptide hormone is a novel observation that couples changes in extracellular environment with endocrine signaling. Understanding this can potentially reveal the complex role of PTH signaling in bone remodeling and improve the PTH(1-34) treatment for osteoporosis.