Isolated Follicles Enriched for Centroblasts and Lacking t(14;18)/BCL2 in Lymphoid Tissue: Diagnostic and Clinical Implications.

We sought to address the significance of isolated follicles that exhibit atypical morphologic features that may be mistaken for lymphoma in a background of reactive lymphoid tissue. Seven cases that demonstrated centroblast-predominant isolated follicles and absent BCL2 staining in otherwise-normal lymph nodes were studied. Four of seven cases showed clonal B-cell proliferations amid a polyclonal B cell background; all cases lacked the IGH-BCL2 translocation and BCL2 protein expression. Although three patients had invasive breast carcinoma at other sites, none were associated with systemic lymphoma up to 44 months after diagnosis. The immunoarchitectural features of these highly unusual cases raise the question of whether a predominance of centroblasts and/or absence of BCL2 expression could represent a precursor lesion or atypical reactive phenomenon. Differentiating such cases from follicular lymphoma or another mimic is critical, lest patients with indolent proliferations be exposed to unnecessarily aggressive treatment.

The genetic basis and expanding role of molecular analysis in the diagnosis, prognosis, and therapeutic design for myelodysplastic syndromes.

The myelodysplastic syndromes (MDS) are clonal hematopoietic stem cell disorders of ineffective hematopoiesis that characteristically demonstrate peripheral blood cytopenia, bone marrow hypercellularity, and morphologically defined dysplasia of one or more hematopoietic lineages. Classical metaphase cytogenetics and judicious use of fluorescence in situ hybridization play central roles in the contemporary diagnosis and classification of MDS. An abundance of recent molecular studies are beginning to delineate additional genetic and epigenetic aberrations associated with these disorders. These alterations affect diagnosis, prognosis, and therapy, and with this understanding classification systems are evolving from a primarily hematological and morphological basis toward a multifactorial appreciation that includes histomorphology, metaphase cytogenetics, and directed molecular studies. In the present health-care environment, it is critical to develop a cost-effective, efficient testing strategy that maximizes the diagnostic potential of even limited specimens. Here, we briefly review the classical genetic approach to MDS, outline exciting new advances in the molecular understanding of this heterogeneous group of hematological neoplasms, and discuss how these advances are driving the evolution of classification and prognostic systems. Rapidly growing understanding of the genetic basis of MDS holds much promise for testing, and here we provide a frame of reference for discussion of current testing protocols and for addressing testing modalities likely to enter clinical practice in the near future.

MITF accurately highlights epidermal melanocytes in atypical intraepidermal melanocytic proliferations.

Atypical intraepidermal melanocytic proliferations (AIMP) have random cytologic atypia and other histologic features that are concerning for malignancy and often require immunohistochemistry to differentiate from melanoma in situ. Immunostaining with S100, Melan-A, and microphthalmia-associated transcription factor (MITF) was performed for 49 morphologically well-characterized AIMP lesions. The percentage of cells in the basal layer of the epidermis that were identified as melanocytes by immunohistochemistry was compared with the percentage observed by morphology on hematoxylin and eosin staining, which is the gold standard stain for identifying cytologic atypia within an AIMP. Melan-A estimated the highest percentage of melanocytes and S100 the fewest in 47 of the 49 lesions examined. The estimated percentage of melanocytes was 23.3% (95% confidence interval: 18.6-28.1; P < 0.001) higher for Melan-A compared with hematoxylin and eosin staining. Melanocyte estimates were similar for hematoxylin and eosin and MITF (P = 0.15) although S100 estimated 21.8% (95% confidence interval: -27.2 to -16.4; P < 0.001) fewer melanocytes than hematoxylin and eosin. Melan-A staining produces higher estimates of epidermal melanocytes than S100 and MITF, which may increase the likelihood of diagnosing melanoma in situ. In contrast, melanoma in situ may be underdiagnosed with the use of S100, which results in lower estimates of melanocytes than the other 2 immunostains. Therefore, the best immunohistochemical marker for epidermal melanocytes is MITF.

Terminal myeloid differentiation in vivo is induced by FLT3 inhibition in FLT3/ITD AML.

A hallmark of cancer is the disruption of differentiation within tumor cells. Internal tandem duplication mutations of the FLT3 kinase (FLT3/ITD) occur commonly in acute myeloid leukemia (AML) and are associated with poor survival, leading to efforts to develop FLT3 kinase inhibitors. However, FLT3 inhibitors have thus far met with limited success, inducing only a clearance of peripheral blasts with minimal BM responses. Quizartinib is a novel potent and selective FLT3 inhibitor currently being studied in clinical trials. In 13 of 14 FLT3/ITD AML patients with normal karyotype treated with quizartinib, we observed terminal myeloid differentiation of BM blasts in association with a clinical differentiation syndrome. The single patient whose blasts failed to differentiate had a preexisting C/EBPα mutation and another developed a C/EBPα mutation at disease progression, suggesting a mechanism of resistance to FLT3 inhibition. In vitro, in primary blasts cocultured with human BM stroma, FLT3 inhibition with quizartinib induced cell-cycle arrest and differentiation rather than apoptosis. The present study is the first description of terminal differentiation of cancer cells in patients treated with a tyrosine kinase inhibitor. These data highlight the importance of the differentiation block in the patho-genesis of AML.

Structural basis for the preferential recognition of immature flaviviruses by a fusion-loop antibody.

Flaviviruses are a group of human pathogens causing severe encephalitic or hemorrhagic diseases that include West Nile, dengue and yellow fever viruses. Here, using X-ray crystallography we have defined the structure of the flavivirus cross-reactive antibody E53 that engages the highly conserved fusion loop of the West Nile virus envelope glycoprotein. Using cryo-electron microscopy, we also determined that E53 Fab binds preferentially to spikes in noninfectious, immature flavivirions but is unable to bind significantly to mature virions, consistent with the limited solvent exposure of the epitope. We conclude that the neutralizing impact of E53 and likely similar fusion-loop-specific antibodies depends on its binding to the frequently observed immature component of flavivirus particles. Our results elucidate how fusion-loop antibodies, which comprise a significant fraction of the humoral response against flaviviruses, can function to control infection without appreciably recognizing mature virions. As these highly cross-reactive antibodies are often weakly neutralizing they also may contribute to antibody-dependent enhancement and flavi virus pathogenesis thereby complicating development of safe and effective vaccines.

Binding of a neutralizing antibody to dengue virus alters the arrangement of surface glycoproteins.

The monoclonal antibody 1A1D-2 has been shown to strongly neutralize dengue virus serotypes 1, 2 and 3, primarily by inhibiting attachment to host cells. A crystal structure of its antigen binding fragment (Fab) complexed with domain III of the viral envelope glycoprotein, E, showed that the epitope would be partially occluded in the known structure of the mature dengue virus. Nevertheless, antibody could bind to the virus at 37 degrees C, suggesting that the virus is in dynamic motion making hidden epitopes briefly available. A cryo-electron microscope image reconstruction of the virus:Fab complex showed large changes in the organization of the E protein that exposed the epitopes on two of the three E molecules in each of the 60 icosahedral asymmetric units of the virus. The changes in the structure of the viral surface are presumably responsible for inhibiting attachment to cells.

The stoichiometry of antibody-mediated neutralization and enhancement of West Nile virus infection.

Antibody binding to the icosahedral arrangement of envelope proteins on the surface of flaviviruses can result in neutralization or enhancement of infection. We evaluated how many antibodies must bind to a given epitope on West Nile virus (WNV) to achieve neutralization. The most potent monoclonal antibodies (mAbs) block infection at concentrations that result in low occupancy of accessible sites on the virion, with neutralization occurring when as few as 30 of 180 envelope proteins are bound. In contrast, weakly neutralizing mAbs recognize fewer sites on the virion and require almost complete occupancy to inhibit WNV infection. For all mAbs studied, enhancement of infection is possible in cells bearing activating Fc-gamma receptors when the number of mAbs docked to the virion is not sufficient for neutralization. Thus, neutralization is best described by a model requiring "multiple hits" with the cumulative functional outcome determined by interplay between antibody affinity and epitope accessibility.

Type- and subcomplex-specific neutralizing antibodies against domain III of dengue virus type 2 envelope protein recognize adjacent epitopes.

Neutralization of flaviviruses in vivo correlates with the development of an antibody response against the viral envelope (E) protein. Previous studies demonstrated that monoclonal antibodies (MAbs) against an epitope on the lateral ridge of domain III (DIII) of the West Nile virus (WNV) E protein strongly protect against infection in animals. Based on X-ray crystallography and sequence analysis, an analogous type-specific neutralizing epitope for individual serotypes of the related flavivirus dengue virus (DENV) was hypothesized. Using yeast surface display of DIII variants, we defined contact residues of a panel of type-specific, subcomplex-specific, and cross-reactive MAbs that recognize DIII of DENV type 2 (DENV-2) and have different neutralizing potentials. Type-specific MAbs with neutralizing activity against DENV-2 localized to a sequence-unique epitope on the lateral ridge of DIII, centered at the FG loop near residues E383 and P384, analogous in position to that observed with WNV-specific strongly neutralizing MAbs. Subcomplex-specific MAbs that bound some but not all DENV serotypes and neutralized DENV-2 infection recognized an adjacent epitope centered on the connecting A strand of DIII at residues K305, K307, and K310. In contrast, several MAbs that had poor neutralizing activity against DENV-2 and cross-reacted with all DENV serotypes and other flaviviruses recognized an epitope with residues in the AB loop of DIII, a conserved region that is predicted to have limited accessibility on the mature virion. Overall, our experiments define adjacent and structurally distinct epitopes on DIII of DENV-2 which elicit type-specific, subcomplex-specific, and cross-reactive antibodies with different neutralizing potentials.

Induction of epitope-specific neutralizing antibodies against West Nile virus.

Previous studies have established that an epitope on the lateral ridge of domain III (DIII-lr) of West Nile virus (WNV) envelope (E) protein is recognized by strongly neutralizing type-specific antibodies. In contrast, an epitope against the fusion loop in domain II (DII-fl) is recognized by flavivirus cross-reactive antibodies with less neutralizing potential. Using gain- and loss-of-function E proteins and wild-type and variant WNV reporter virus particles, we evaluated the expression pattern and activity of antibodies against the DIII-lr and DII-fl epitopes in mouse and human serum after WNV infection. In mice, immunoglobulin M (IgM) antibodies to the DIII-lr epitope were detected at low levels at day 6 after infection. However, compared to IgG responses against other epitopes in DI and DII, which were readily detected at day 8, the development of IgG against DIII-lr epitope was delayed and did not appear consistently until day 15. This late time point is notable since almost all death after WNV infection in mice occurs by day 12. Nonetheless, at later time points, DIII-lr antibodies accumulated and comprised a significant fraction of the DIII-specific IgG response. In sera from infected humans, DIII-lr antibodies were detected at low levels and did not correlate with clinical outcome. In contrast, antibodies to the DII-fl were detected in all human serum samples and encompassed a significant percentage of the anti-E protein response. Our experiments suggest that the highly neutralizing DIII-lr IgG antibodies have little significant role in primary infection and that the antibody response of humans may be skewed toward the induction of cross-reactive, less-neutralizing antibodies.

Antibody recognition and neutralization determinants on domains I and II of West Nile Virus envelope protein.

Previous studies have demonstrated that monoclonal antibodies (MAbs) against an epitope on the lateral surface of domain III (DIII) of the West Nile virus (WNV) envelope (E) strongly protect against infection in animals. Herein, we observed significantly less efficient neutralization by 89 MAbs that recognized domain I (DI) or II (DII) of WNV E protein. Moreover, in cells expressing Fc gamma receptors, many of the DI- and DII-specific MAbs enhanced infection over a broad range of concentrations. Using yeast surface display of E protein variants, we identified 25 E protein residues to be critical for recognition by DI- or DII-specific neutralizing MAbs. These residues cluster into six novel and one previously characterized epitope located on the lateral ridge of DI, the linker region between DI and DIII, the hinge interface between DI and DII, and the lateral ridge, central interface, dimer interface, and fusion loop of DII. Approximately 45% of DI-DII-specific MAbs showed reduced binding with mutations in the highly conserved fusion loop in DII: 85% of these (34 of 40) cross-reacted with the distantly related dengue virus (DENV). In contrast, MAbs that bound the other neutralizing epitopes in DI and DII showed no apparent cross-reactivity with DENV E protein. Surprisingly, several of the neutralizing epitopes were located in solvent-inaccessible positions in the context of the available pseudoatomic model of WNV. Nonetheless, DI and DII MAbs protect against WNV infection in mice, albeit with lower efficiency than DIII-specific neutralizing MAbs.

Crystal structure of the West Nile virus envelope glycoprotein.

The envelope glycoprotein (E) of West Nile virus (WNV) undergoes a conformational rearrangement triggered by low pH that results in a class II fusion event required for viral entry. Herein we present the 3.0-A crystal structure of the ectodomain of WNV E, which reveals insights into the flavivirus life cycle. We found that WNV E adopts a three-domain architecture that is shared by the E proteins from dengue and tick-borne encephalitis viruses and forms a rod-shaped configuration similar to that observed in immature flavivirus particles. Interestingly, the single N-linked glycosylation site on WNV E is displaced by a novel alpha-helix, which could potentially alter lectin-mediated attachment. The localization of histidines within the hinge regions of E implicates these residues in pH-induced conformational transitions. Most strikingly, the WNV E ectodomain crystallized as a monomer, in contrast to other flavivirus E proteins, which have crystallized as antiparallel dimers. WNV E assembles in a crystalline lattice of perpendicular molecules, with the fusion loop of one E protein buried in a hydrophobic pocket at the DI-DIII interface of another. Dimeric E proteins pack their fusion loops into analogous pockets at the dimer interface. We speculate that E proteins could pivot around the fusion loop-pocket junction, allowing virion conformational transitions while minimizing fusion loop exposure.

West Nile virus in complex with the Fab fragment of a neutralizing monoclonal antibody.

Flaviviruses, such as West Nile virus (WNV), are significant human pathogens. The humoral immune response plays an important role in the control of flavivirus infection and disease. The structure of WNV complexed with the Fab fragment of the strongly neutralizing mAb E16 was determined to 14.5-Angstrom resolution with cryo-electron microscopy. E16, an antibody with therapeutic potential, binds to domain III of the WNV envelope glycoprotein. Because of steric hindrance, Fab E16 binds to only 120 of the 180 possible binding sites on the viral surface. Fitting of the previously determined x-ray structure of the Fab-domain III complex into the cryo-electron microscopy density required a change of the elbow angle between the variable and constant domains of the Fab. The structure suggests that the E16 antibody neutralizes WNV by blocking the initial rearrangement of the E glycoprotein before fusion with a cellular membrane.

Antibodies against West Nile Virus nonstructural protein NS1 prevent lethal infection through Fc gamma receptor-dependent and -independent mechanisms.

The flavivirus nonstructural protein NS1 is a highly conserved secreted glycoprotein that does not package with the virion. Immunization with NS1 elicits a protective immune response against yellow fever, dengue, and tick-borne encephalitis flaviviruses through poorly defined mechanisms. In this study, we purified a recombinant, secreted form of West Nile virus (WNV) NS1 glycoprotein from baculovirus-infected insect cells and generated 22 new NS1-specific monoclonal antibodies (MAbs). By performing competitive binding assays and expressing truncated NS1 proteins on the surface of yeast (Saccharomyces cerevisiae) and in bacteria, we mapped 21 of the newly generated MAbs to three NS1 fragments. Prophylaxis of C57BL/6 mice with any of four MAbs (10NS1, 14NS1, 16NS1, and 17NS1) strongly protected against lethal WNV infection (75 to 95% survival, respectively) compared to saline-treated controls (17% survival). In contrast, other anti-NS1 MAbs of the same isotype provided no significant protection. Notably, 14NS1 and 16NS1 also demonstrated marked efficacy as postexposure therapy, even when administered as a single dose 4 days after infection. Virologic analysis showed that 17NS1 protects at an early stage in infection through a C1q-independent and Fc gamma receptor-dependent pathway. Interestingly, 14NS1, which maps to a distinct region on NS1, protected through a C1q- and Fc gamma receptor-independent mechanism. Overall, our data suggest that distinct regions of NS1 can elicit protective humoral immunity against WNV through different mechanisms.

Structural basis of West Nile virus neutralization by a therapeutic antibody.

West Nile virus is a mosquito-borne flavivirus closely related to the human epidemic-causing dengue, yellow fever and Japanese encephalitis viruses. In establishing infection these icosahedral viruses undergo endosomal membrane fusion catalysed by envelope glycoprotein rearrangement of the putative receptor-binding domain III (DIII) and exposure of the hydrophobic fusion loop. Humoral immunity has an essential protective function early in the course of West Nile virus infection. Here, we investigate the mechanism of neutralization by the E16 monoclonal antibody that specifically binds DIII. Structurally, the E16 antibody Fab fragment engages 16 residues positioned on four loops of DIII, a consensus neutralizing epitope sequence conserved in West Nile virus and distinct in other flaviviruses. The E16 epitope protrudes from the surface of mature virions in three distinct environments, and docking studies predict Fab binding will leave five-fold clustered epitopes exposed. We also show that E16 inhibits infection primarily at a step after viral attachment, potentially by blocking envelope glycoprotein conformational changes. Collectively, our results suggest that a vaccine strategy targeting the dominant DIII epitope may elicit safe and effective immune responses against flaviviral diseases.

Toward the development of peptide nanofilaments and nanoropes as smart materials.

Protein design studies using coiled coils have illustrated the potential of engineering simple peptides to self-associate into polymers and networks. Although basic aspects of self-assembly in protein systems have been demonstrated, it remains a major challenge to create materials whose large-scale structures are well determined from design of local protein-protein interactions. Here, we show the design and characterization of a helical peptide, which uses phased hydrophobic interactions to drive assembly into nanofilaments and fibrils ("nanoropes"). Using the hydrophobic effect to drive self-assembly circumvents problems of uncontrolled self-assembly seen in previous approaches that used electrostatics as a mode for self-assembly. The nanostructures designed here are characterized by biophysical methods including analytical ultracentrifugation, dynamic light scattering, and circular dichroism to measure their solution properties, and atomic force microscopy to study their behavior on surfaces. Additionally, the assembly of such structures can be predictably regulated by using various environmental factors, such as pH, salt, other molecular crowding reagents, and specifically designed "capping" peptides. This ability to regulate self-assembly is a critical feature in creating smart peptide biomaterials.

Development of a humanized monoclonal antibody with therapeutic potential against West Nile virus.

Neutralization of West Nile virus (WNV) in vivo correlates with the development of an antibody response against the viral envelope (E) protein. Using random mutagenesis and yeast surface display, we defined individual contact residues of 14 newly generated monoclonal antibodies against domain III of the WNV E protein. Monoclonal antibodies that strongly neutralized WNV localized to a surface patch on the lateral face of domain III. Convalescent antibodies from individuals who had recovered from WNV infection also detected this epitope. One monoclonal antibody, E16, neutralized 10 different strains in vitro, and showed therapeutic efficacy in mice, even when administered as a single dose 5 d after infection. A humanized version of E16 was generated that retained antigen specificity, avidity and neutralizing activity. In postexposure therapeutic trials in mice, a single dose of humanized E16 protected mice against WNV-induced mortality, and may therefore be a viable treatment option against WNV infection in humans.