Development and Characterization of Monoclonal Antibodies Specific for Mouse and Human Fcγ Receptors.

FcγRs are key regulators of the immune response, capable of binding to the Fc portion of IgG Abs and manipulating the behavior of numerous cell types. Through a variety of receptors, isoforms, and cellular expression patterns, they are able to fine-tune and direct appropriate responses. Furthermore, they are key determinants of mAb immunotherapy, with mAb isotype and FcγR interaction governing therapeutic efficacy. Critical to understanding the biology of this complex family of receptors are reagents that are robust and highly specific for each receptor. In this study, we describe the development and characterization of mAb panels specific for both mouse and human FcγR for use in flow cytometry, immunofluorescence, and immunocytochemistry. We highlight key differences in expression between the two species and also patterns of expression that will likely impact on immunotherapeutic efficacy and translation of therapeutic agents from mouse to clinic.

Fcγ receptor dependency of agonistic CD40 antibody in lymphoma therapy can be overcome through antibody multimerization.

Immunomodulatory mAbs, led by the anti-CTLA4 mAb ipilimumab, are an exciting new class of drugs capable of promoting anticancer immunity and providing durable control of some tumors. Close analysis of a number of agents has revealed a critical yet variable role for Fcγ receptors in their efficacy. In this article, we reveal that agonistic anti-CD40 mAbs have an absolute requirement for cross-linking by inhibitory FcγRIIB when used systemically to treat established BCL1 syngeneic lymphoma, and therapy is lost when using a mouse IgG2a mAb not cross-linked by FcγRIIB. Furthermore, in FcγRIIB-deficient mice the lymphoma itself can provide FcγRIIB to cross-link anti-CD40 on neighboring cells, and only when this is blocked does therapy fail. The dependence on FcγRIIB for immunostimulatory activity was not absolute, however, because when anti-CD40 mAbs were administered systemically with the TLR3 agonist polyinosinic:polycytidylic acid or were given subcutaneously, activatory FcγR could also provide cross-linking. Using this mechanistic insight, we designed multimeric forms of anti-CD40 mAb with intrinsic FcγR-independent activity that were highly effective in the treatment of lymphoma-bearing mice. In conclusion, FcγR-independent anti-CD40 activation is a viable strategy in vivo. These findings have important translational implications, as humans, unlike mice, do not have IgG that binds strongly to FcγRIIB; therefore FcγR-independent derivatives represent an attractive therapeutic option.

Specificity of bispecific T cell receptors and antibodies targeting peptide-HLA.

Tumor-associated peptide-human leukocyte antigen complexes (pHLAs) represent the largest pool of cell surface-expressed cancer-specific epitopes, making them attractive targets for cancer therapies. Soluble bispecific molecules that incorporate an anti-CD3 effector function are being developed to redirect T cells against these targets using 2 different approaches. The first achieves pHLA recognition via affinity-enhanced versions of natural TCRs (e.g., immune-mobilizing monoclonal T cell receptors against cancer [ImmTAC] molecules), whereas the second harnesses an antibody-based format (TCR-mimic antibodies). For both classes of reagent, target specificity is vital, considering the vast universe of potential pHLA molecules that can be presented on healthy cells. Here, we made use of structural, biochemical, and computational approaches to investigate the molecular rules underpinning the reactivity patterns of pHLA-targeting bispecifics. We demonstrate that affinity-enhanced TCRs engage pHLA using a comparatively broad and balanced energetic footprint, with interactions distributed over several HLA and peptide side chains. As ImmTAC molecules, these TCRs also retained a greater degree of pHLA selectivity, with less off-target activity in cellular assays. Conversely, TCR-mimic antibodies tended to exhibit binding modes focused more toward hot spots on the HLA surface and exhibited a greater degree of crossreactivity. Our findings extend our understanding of the basic principles that underpin pHLA selectivity and exemplify a number of molecular approaches that can be used to probe the specificity of pHLA-targeting molecules, aiding the development of future reagents.

Conformation of the human immunoglobulin G2 hinge imparts superagonistic properties to immunostimulatory anticancer antibodies.

Monoclonal antibody (mAb) drugs that stimulate antitumor immunity are transforming cancer treatment but require optimization for maximum clinical impact. Here, we show that, unlike other immunoglobulin isotypes, human IgG2 (h2) imparts FcγR-independent agonistic activity to immune-stimulatory mAbs such as anti-CD40, -4-1BB, and -CD28. Activity is provided by a subfraction of h2, h2B, that is structurally constrained due its unique arrangement of hinge region disulfide bonds. Agonistic activity can be transferred from h2 to h1 by swapping their hinge and CH1 domains, and substitution of key hinge and CH1 cysteines generates homogenous h2 variants with distinct agonistic properties. This provides the exciting opportunity to engineer clinical reagents with defined therapeutic activity regardless of FcγR expression levels in the local microenvironment.

Acute regulation of mUT-A3 urea transporter expressed in a MDCK cell line.

Renal facilitative urea transporters play a vital role in the urinary concentrating mechanism. UT-A3 is a phloretin-sensitive urea transporter that in the mouse is expressed on the basolateral membrane of renal inner medullary collecting duct (IMCD) cells. In this study, we engineered a Madin-Darby canine kidney (MDCK) I cell line that stably expresses mouse UT-A3 (MDCK-mUT-A3). Immunoblotting using the UT-A-targeted antibody ML446 detected a approximately 40-kDa signal in MDCK-mUT-A3 protein that corresponds to mUT-A3. Using cultured epithelial monolayers, radioactive (14)C-urea flux experiments determined that basolateral urea transport was no different between MDCK-mUT-A3 and control MDCK-FLZ cells under basal conditions [not significant (NS), ANOVA]. However, exposure to arginine vasopressin (AVP) significantly stimulated basolateral urea flux in MDCK-mUT-A3 monolayers (P < 0.05, ANOVA), while it had no effect in control MDCK-FLZ monolayers (NS, ANOVA). The AVP-stimulated basolateral urea transport in MDCK-mUT-A3 was inhibited by 1,3 dimethyl urea (P < 0.05, ANOVA) or phloretin (P < 0.05, ANOVA), both known inhibitors of facilitative urea transporters. MDCK-mUT-A3 basolateral urea flux was also stimulated by increasing intracellular levels of cAMP, via forskolin (P < 0.05, ANOVA), or intracellular calcium, via ATP (P < 0.05, ANOVA). Finally, 1-h preincubation with a specific PKA inhibitor, H89, significantly inhibited the increase in urea transport produced by AVP (P < 0.05, ANOVA). In conclusion, we have produced the first renal cell line to stably express the mUT-A3 urea transporter. Our results indicate that mUT-A3 is acutely regulated by AVP, via a PKA-dependent pathway. These findings have important implications for the regulation of urea transport in the renal IMCD and the urinary concentrating mechanism.

Urea flux across MDCK-mUT-A2 monolayers is acutely sensitive to AVP, cAMP, and [Ca2+]i.

In this study, we engineered a Madin-Darby canine kidney (MDCK) type I cell line to stably express the mouse urea transporter UT-A2. Monolayers of MDCK-mUT-A2 cells had a basal phloretin-inhibitable urea permeability of 8.4x10(-6)+/-0.3 cm/s. Treatment of MDCK-mUT-A2 monolayers with AVP led to a rapid dose-dependent increase in trans-monolayer phloretin-inhibitable urea flux. The temporal pattern of response was markedly different from that observed for MDCK cells expressing rat UT-A1. Exposure of MDCK-mUT-A2 cells to either 10 microM forskolin or 250 microM 8-bromo cAMP also increased urea flux rate. Inclusion of the PKA inhibitor H89 (10 microM) had no effect on the forskolin-stimulated increase in urea flux across MDCK-mUT-A2 monolayers. Treatment with either 10 microM CPA or 1 mM ATP also caused an increase in UT-A2-mediated urea flux, although these responses where transient compared with those induced by AVP or elevated cAMP. Taken together, these results show for the first time that UT-A2 is acutely sensitive to AVP, cAMP, or increased intracellular calcium.

Characterization of a human colonic cDNA encoding a structurally novel urea transporter, hUT-A6.

Two closely related genes, UT-A (Slc14a2) and UT-B (Slc14a1), encode specialized transporter proteins that modulate the movement of urea across cell membranes. In this article, we report the characterization of a cDNA isolated from human colonic mucosa encoding a novel UT-A urea transporter, hUT-A6. The encoded protein is 235 amino acids (aa) in length, making it the smallest UT-A member characterized. On the basis of previous structural predictions, hUT-A6 is structurally unique in that it consists of a single hydrophobic core flanked by hydrophilic NH(2)- and COOH-terminal domains. The transcript encoding hUT-A6 contains a novel 129-bp exon, exon 5a, which, as a result of alternative splicing, introduces a unique 19-aa segment and a stop codon. Functionally, the protein transports urea, and this activity is inhibited by phloretin. Interestingly, despite the lack of a protein kinase A (PKA) consensus site [RK](2)-X-[ST], transport of urea by hUT-A6 is stimulated by PKA agonists. Deletion of the two PKA consensus sites from murine UT-A3 (mUT-A3) did not affect the stimulatory response of PKA agonists, which, together with the lack of PKA consensus sites in hUT-A6, indicates that regulation of hUT-A6 and mUT-A3 is not mediated through a classic PKA phosphorylation consensus.