Generation of anti-idiotypic monoclonal antibodies recognizing vasopressin receptors in cultured cells and kidney sections

Recombinant Anti-idiotypic Antibodies in Ligand Binding Assays for Antibody Drug Development

Sensitive and reproducible pharmacokinetic (PK) assays and immunogenicity assessment are required as part of the complex and lengthy development process for biotherapeutic proteins. Ligand binding assays (LBAs) are included in a range of approaches applied to understand the nature and properties of the drug as well as the induction of anti-drug antibodies (ADA) against the therapeutic, which can cause adverse events and loss of efficacy. Currently, most biotherapeutics are monoclonal human or humanized antibodies. Anti-idiotypic antibodies, targeting the idiotopic determinants of individual antibody drugs are recognized as perfect reagents for such LBAs. Here we describe the typical setups for these assays and how different types of anti-biotherapeutic antibodies can be used to establish selective and sensitive assays.

Functional role of the NPxxY motif in internalization of the type 2 vasopressin receptor in LLC-PK1 cells

Interaction of the type 2 vasopressin receptor (V2R) with hormone causes desensitization and internalization. To study the role of the V2R NPxxY motif (which is involved in the clathrin-mediated endocytosis of several other receptors) in this process, we expressed FLAG-tagged wild-type V2R and a Y325F mutant V2R in LLC-PK1a epithelial cells that have low levels of endogenous V2R. Both proteins had a similar apical (35%) and basolateral (65%) membrane distribution. Substitution of Tyr325 with Phe325 prevented ligand-induced internalization of V2R determined by [3H]AVP binding and immunofluorescence but did not prevent ligand binding or signal transduction via adenylyl cyclase. Desensitization and resensitization of the V2R-Y325F mutation occurred independently of internalization. The involvement of clathrin in V2R downregulation was also shown by immunogold electron microscopy. We conclude that the NPxxY motif of the V2R is critically involved in receptor downregulation via clathrin-mediated internalization. However, this motif is not essential for the apical/basolateral sorting and polarized distribution of the V2R in LLC-PK1a cells or for adenylyl cyclase-mediated signal transduction.

Biochemistry of vasopressin fragments

Vasopressin (VP) undergoes a step-wise aminopeptidase conversion process in the brain, leading to accumulation of several metabolites. Some of these metabolites, in particular [pGlu4,Cyt6]VP 4-9 and 4-8, show behavioral effects comparable to VP, but are more potent and selective than VP. Most data favor the existence of a separate receptor for the VP metabolites distinct of the classical VP and oxytocin receptors, although its identity has remained obscure thus far. The characterization of this receptor is a major challenge to understand how the brain VP system generates and regulates divers central functions.

Vasopressin and sensory circumventricular organs

The subfornical organ, the area postrema and the organum vasculosum of the lamina terminalis are considered to be sensory circumventricular organs as they contain neuronal somata which are located outside the blood-brain barrier and are thus capable of serving as 'sensors' for blood-borne humoral messengers. The endocrine hormone, vasopressin (VP), not only causes strong antidiuresis by acting on the kidney, but also exerts centrally mediated effects as a neuromodulator. Several lines of evidence suggest that VP can influence regulatory functions mediated by the sensory circumventricular organs, since vasopressinergic somata and terminals as well as VP receptors have been reposted to be present in these structures. These biochemical prerequisites offer the possibility that blood-borne VP might on the one hand act as a feedback signal from the periphery and, on the other hand, synaptically released or locally produced VP could modulate the known functions of sensory circumventricular organs, such as thirst, fever or cardiovascular regulation. This review focuses on the possible physiological relevance of VP acting on sensory circumventricular organs in view of recent evidence obtained from biochemical and electrophysiological studies at the cellular level.

Immunolocalization of V1 vasopressin receptors in the rat kidney using anti-receptor antibodies

By using immunocytochemical techniques we have been able to localize the V1 vasopressin receptor in the rat kidney. Immunoblotting using an antiserum raised against an affinity-purified vasopressin receptor showed a 55,000 daltons protein band that has a molecular mass similar to that of the liver V1 vasopressin receptor, as demonstrated by cross-linking studies. Immunoblotting of the antibody showed a band of 55,000 daltons in A-10 cells, which contains the V1 subtype, whereas it did not stain LLC-PK1 cells, which possess the V2 subtype, showing that the antibody recognizes the V1 vasopressin receptor. The immunostaining of kidney sections with this antiserum showed a strong reaction of the connecting tubules and cortical and medullary collecting ducts. The immunostaining pattern of connecting tubule and collecting duct cells was different, that is, the former showed a staining of both the apical and basal plasma membrane but also in the cytoplasm, whereas the latter showed a strong reaction mainly in the basolateral membrane. Immunostaining of consecutive serial sections with an antiserum raised against tissue kallikrein, an enzyme present exclusively in connecting tubules, and with the anti-receptor serum allowed us to show, for the first time, the presence of the vasopressin receptor in the connecting tubule cells and their absence in intercalated cells, the other cell type present in connecting tubules. These findings support experiments carried in the eighties on the release of renal tissue kallikrein by AVP.

Vasopressin anti-idiotypic antibody staining in the rat brain: colocalization with [35S] [pGlu4, Cyt6]AVP(4-9) binding sites

Vasopressin and its fragment peptides such as [pGlu4, Cyt6]AVP(4-9) (AVP(4-9) represent putative neuromodulators within central nervous homeostatic, memory and behavioural circuits. To localize their central receptor systems, the previously characterized monoclonal anti-idiotypic antibody mAb 237 was employed in immunocytological investigations of rat brain tissue sections. This antibody was raised to the monoclonal idiotypic anti-AVP antibody mAb 113 which preferentially binds to the acyclic C-terminal portion of the AVP molecule and is therefore also capable of binding the naturally occurring AVP(4-9) fragment. Immunoreactive magnocellular neurones were detected in the AVP-synthesizing supraoptic but not paraventricular nuclei. Dense staining was observed within circumventricular organs lacking a blood-brain barrier (BBB). These structures include the subfornical organ, the organum vasculosum laminae terminalis, the internal layer of the median eminence, the body of the pineal gland, the choroid plexus and the area postrema, where immunoreactivity was found on capillaries, neurones and fibres. Further staining was found in the nucleus of the solitari tract and the arcuate nucleus, endowed with a leaky BBB. Distinct cell patches in the ependymal lining of the third ventricle as well as dendritic processes of juxtaependymal neurones were labelled by the anti-idiotypic antibody mAb 237. The observed staining pattern did not parallel that obtained in autoradiographic studies performed using either radiolabelled AVP or a V1-receptor antagonist, but that found with the [35S]-labelled AVP(4-9) fragment. Using [35S]-labelled AVP(4-9) fragment, specific high density binding sites could be localized autoradiographically in structures within and outside the BBB, in complete agreement with the anti-idiotypic immunoreactivity. Since the anti-idiotypic methodology is based on transfer of complementary structures, and the epitope recognized by the corresponding idiotypic antibody resembles the sequence of AVP(4-9), the anti-idiotypic antibodies might recognize the AVP(4-9) receptor with high affinity.

Renal and central vasopressin receptors: immunocytochemical localization

Employing an anti-vasopressin monoclonal antibody for immunization, anti-idiotypic monoclonal antibodies were obtained which induced plasminogen activator production in the renal epithelial cell line LLC-PK1. The anti-idiotypic antibodies were employed to visualize vasopressin receptors on LLC-PK1 and A7r5 smooth muscle cells by immunofluorescence. All results indicated specificity of the anti-idiotypes for both V1 and V2 vasopressin receptor subtypes. These antibodies were used for immunohistochemical localization of vasopressin receptors in rat and bovine kidney preparations. In accordance with earlier physiological and biochemical observations, vasopressin receptors were detected predominantly in collecting ducts in cortex and medulla. On the cellular level, a differential staining pattern was observed. On rat brain tissue sections, dense staining was observed within various circumventricular organs. The staining pattern corresponded to that obtained in autoradiographic studies with labeled AVP(4-9) fragment peptide and differed from the distribution of binding sites for labeled vasopressin or V1 antagonists.