Intranasal immunization with the recombinant measles virus encoding the spike protein of SARS-CoV-2 confers protective immunity against COVID-19 in hamsters

BACKGROUND

As the nasal mucosa is the initial site of infection for COVID-19, intranasal vaccines are more favorable than conventional vaccines. In recent clinical studies, intranasal immunization has been shown to generate higher neutralizing antibodies; however, there is a lack of evidence on sterilizing immunity in the upper airway. Previously, we developed a recombinant measles virus encoding the spike protein of SARS-CoV-2 (rMeV-S), eliciting humoral and cellular immune responses against SARS-CoV-2.

OBJECTIVES

In this study, we aim to provide an experiment on nasal vaccines focusing on a measles virus platform as well as injection routes.

STUDY DESIGN

Recombinant measles viruses expressing rMeV-S were prepared, and 5 × 105 PFUs of rMeV-S were administered to Syrian golden hamsters via intramuscular or intranasal injection. Subsequently, the hamsters were challenged with inoculations of 1 × 105 PFUs of SARS-CoV-2 and euthanized 4 days post-infection. Neutralizing antibodies and RBD-specific IgG in the serum and RBD-specific IgA in the bronchoalveolar lavage fluid (BALF) were measured, and SARS-CoV-2 clearance capacity was determined via quantitative reverse-transcription PCR (qRT-PCR) analysis and viral titer measurement in the upper respiratory tract and lungs. Immunohistochemistry and histopathological examinations of lung samples from experimental hamsters were conducted.

RESULTS

The intranasal immunization of rMeV-S elicits protective immune responses and alleviates virus-induced pathophysiology, such as body weight reduction and lung weight increase in hamsters. Furthermore, lung immunohistochemistry demonstrated that intranasal rMeV-S immunization induces effective SARS-CoV-2 clearance that correlates with viral RNA content, as determined by qRT-PCR, in the lung and nasal wash samples, SARS-CoV-2 viral titers in lung, nasal wash, BALF samples, serum RBD-specific IgG concentration, and RBD-specific IgA concentration in the BALF.

CONCLUSION

An intranasal vaccine based on the measles virus platform is a promising strategy owing to the typical route of infection of the virus, the ease of administration of the vaccine, and the strong immune response it elicits.

mRNA-HPV vaccine encoding E6 and E7 improves therapeutic potential for HPV-mediated cancers via subcutaneous immunization

The E6 and E7 proteins of specific subtypes of human papillomavirus (HPV), including HPV 16 and 18, are highly associated with cervical cancer as they modulate cell cycle regulation. The aim of this study was to investigate the potential antitumor effects of a messenger RNA-HPV therapeutic vaccine (mHTV) containing nononcogenic E6 and E7 proteins. To achieve this, C57BL/6j mice were injected with the vaccine via both intramuscular and subcutaneous routes, and the resulting effects were evaluated. mHTV immunization markedly induced robust T cell-mediated immune responses and significantly suppressed tumor growth in both subcutaneous and orthotopic tumor-implanted mouse model, with a significant infiltration of immune cells into tumor tissues. Tumor retransplantation at day 62 postprimary vaccination completely halted progression in all mHTV-treated mice. Furthermore, tumor expansion was significantly reduced upon TC-1 transplantation 160 days after the last immunization. Immunization of rhesus monkeys with mHTV elicited promising immune responses. The immunogenicity of mHTV in nonhuman primates provides strong evidence for clinical application against HPV-related cancers in humans. All data suggest that mHTV can be used as both a therapeutic and prophylactic vaccine.

mRNA vaccine encoding Gn provides protection against severe fever with thrombocytopenia syndrome virus in mice

We developed a promising mRNA vaccine against severe fever with thrombocytopenia syndrome (SFTS), an infectious disease caused by the SFTS virus that is primarily transmitted through tick bites. Administration of lipid nanoparticle-encapsulated mRNA-Gn successfully induced neutralizing antibodies and T-cell responses in mice. The vaccinated mice were protected against a lethal SFTS virus challenge, suggesting that this mRNA vaccine may be an effective and successful SFTS vaccine candidate.

Analysis of the immunostimulatory effects of cytokine-expressing internal ribosome entry site-based RNA adjuvants and their applications

Developing new adjuvants that can effectively induce both humoral and cellular immune responses while broadening the immune response is of great value. In this study, we aimed to develop GM-CSF- or IL-18-expressing single-stranded RNA (ssRNA) adjuvants based on the encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) and tested their efficacy in combination with ovalbumin (OVA) or inactivated influenza vaccines. Notably, cytokine-expressing RNA adjuvants increased the expression of antigen-presenting cell activation markers. Specifically, GM-CSF-expressing RNA adjuvants increased CD4+T cell responses, while IL-18-expressing RNA adjuvants increased CD8+T cell responses in mice when combined with OVA. In addition, cytokine-expressing RNA adjuvants increased the frequency of polyclonal T cells in combination with the influenza vaccine and reduced the clinical illness scores and weight loss of mice after viral challenge. Collectively, our results suggest that cytokine-expressing RNA adjuvants can be applied to protein-based or inactivated vaccines to increase their efficacy.

Analyzing immune responses to varied mRNA and protein vaccine sequences

In response to the COVID-19 pandemic, different types of vaccines, such as inactive, live-attenuated, messenger RNA (mRNA), and protein subunit, have been developed against SARS-CoV-2. This has unintentionally created a unique scenario where heterologous prime-boost vaccination against a single virus has been administered to a large human population. Here, we aimed to analyze whether the immunization order of vaccine types influences the efficacy of heterologous prime-boost vaccination, especially mRNA and protein-based vaccines. We developed a new mRNA vaccine encoding the hemagglutinin (HA) glycoprotein of the influenza virus using the 3'-UTR and 5'-UTR of muscle cells (mRNA-HA) and tested its efficacy by heterologous immunization with an HA protein vaccine (protein-HA). The results demonstrated higher IgG2a levels and hemagglutination inhibition titers in the mRNA-HA priming/protein-HA boosting (R-P) regimen than those induced by reverse immunization (protein-HA priming/mRNA-HA boosting, P-R). After the viral challenge, the R-P group showed lower virus loads and less inflammation in the lungs than the P-R group did. Transcriptome analysis revealed that the heterologous prime-boost groups had differentially activated immune response pathways, according to the order of immunization. In summary, our results demonstrate that the sequence of vaccination is critical to direct desired immune responses. This study demonstrates the potential of a heterologous vaccination strategy using mRNA and protein vaccine platforms against viral infection.

Heterologous vaccination utilizing viral vector and protein platforms confers complete protection against SFTSV

Severe fever with thrombocytopenia syndrome virus was first discovered in 2009 as the causative agent of severe fever with thrombocytopenia syndrome. Despite its potential threat to public health, no prophylactic vaccine is yet available. This study developed a heterologous prime-boost strategy comprising priming with recombinant replication-deficient human adenovirus type 5 (rAd5) expressing the surface glycoprotein, Gn, and boosting with Gn protein. This vaccination regimen induced balanced Th1/Th2 immune responses and resulted in potent humoral and T cell-mediated responses in mice. It elicited high neutralizing antibody titers in both mice and non-human primates. Transcriptome analysis revealed that rAd5 and Gn proteins induced adaptive and innate immune pathways, respectively. This study provides immunological and mechanistic insight into this heterologous regimen and paves the way for future strategies against emerging infectious diseases.

Recombinant measles virus encoding the spike protein of SARS-CoV-2 efficiently induces Th1 responses and neutralizing antibodies that block SARS-CoV-2 variants

Owing to the rapid spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its variants, the development of effective and safe vaccines has become a priority. The measles virus (MeV) vaccine is an attractive vaccine platform as it has been administered to children for more than 40 years in over 100 countries. In this study, we developed a recombinant MeV expressing the full-length SARS-CoV-2 spike protein (rMeV-S) and tested its efficacy using mouse and hamster models. In hCD46Tg mice, two-dose rMeV-S vaccination induced higher Th1 secretion and humoral responses than one-dose vaccination. Interestingly, neutralizing antibodies induced by one-dose and two-dose rMeV-S immunization effectively blocked the entry of the α, β, γ, and δ variants of SARS-CoV-2. Furthermore, two-dose rMeV-S immunization provided complete protection against SARS-CoV-2 in the hamster model. These results suggest the potential of rMeV-S as a vaccine candidate for targeting SARS-CoV-2 and its variants.

Adjuvant effect of IRES-based single-stranded RNA on melanoma immunotherapy

Background

Adjuvant therapies such as radiation therapy, chemotherapy, and immunotherapy are usually given after cancer surgery to improve the survival of cancer patients. However, despite advances in several adjuvant therapies, they are still limited in the prevention of recurrences.

Methods

We evaluated the immunological effects of RNA-based adjuvants in a murine melanoma model. Single-stranded RNA (ssRNA) were constructed based on the cricket paralysis virus (CrPV) internal ribosome entry site (IRES). Populations of immune cells in bone marrow cells and lymph node cells following immunization with CrPV^IRES-ssRNA were determined using flow cytometry. Activated cytokine levels were measured using ELISA and ELISpot. The tumor protection efficacy of CrPV^IRES-ssRNA was analyzed based on any reduction in tumor size or weight, and overall survival.

Results

CrPV^IRES-ssRNA treatment stimulated antigen-presenting cells in the drain lymph nodes associated with activated antigen-specific dendritic cells. Next, we evaluated the expression of CD40, CD86, and XCR1, showing that immunization with CrPV^IRES-ssRNA enhanced antigen presentation by CD8a⁺ conventional dendritic cell 1 (cDC1), as well as activated antigen-specific CD8 T cells. In addition, CrPV^IRES-ssRNA treatment markedly increased the frequency of antigen-specific CD8 T cells and interferon-gamma (IFN-γ) producing cells, which promoted immune responses and reduced tumor burden in melanoma-bearing mice.

Conclusions

This study provides evidence that the CrPV^IRES-ssRNA adjuvant has potential for use in therapeutic cancer vaccines. Moreover, CrPV^IRES-ssRNA possesses protective effects on various cancer cell models.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-10140-2.

Single-stranded RNA adjuvant enhances the efficacy of 10-valent human papilloma virus-like particle vaccine

Following the development of various types of vaccines, the use of adjuvants to boost vaccine efficacy has become a focus of research. Aluminum hydroxide (alum), the most commonly used adjuvant, induces a certain immune response and ensures safety in human trials. However, alum mainly induces only a Th2 response; its Th1 response is weak. Thus, we previously developed a single-stranded ribose nucleic acid (ssRNA) adjuvant that induces a Th1 response through toll-like receptors. Here, we explored whether 10-valent human papilloma virus (HPV)-like particle (VLP) vaccine formulated with ssRNA adjuvant and alum helped enhance immune response and maintained memory response. The mice were immunized intramuscularly twice at 2-week intervals and were inoculated 4 days after the second boost (after about 1 year). Antibody response and T cell activation were measured by Elispot, ELISA using harvested serum and splenocytes. 10-valent HPV VLP vaccine formulated with ssRNA adjuvant and alum increased antigen-specific immune response than alum used alone. It increased each type-specific IgG1/IgG2c titers, and antigen-specific IFN-γ cells. Furthermore, the ssRNA adjuvant with alum induced memory response. In memory response, each type-specific IgG1/IgG2c, IFN-γ, and IL-6 cytokines, and neutralizing antibodies were increased by the ssRNA adjuvant with alum. Overall, the ssRNA adjuvant with alum induced memory responses and balanced Th1/Th2 responses. The ssRNA adjuvant and alum may help to enhanced prophylactic vaccine efficacy. This article is protected by copyright. All rights reserved.

Immunization with RBD-P2 and N protects against SARS-CoV-2 in nonhuman primates

Since the emergence of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), various vaccines are being developed, with most vaccine candidates focusing on the viral spike protein. Here, we developed a previously unknown subunit vaccine comprising the receptor binding domain (RBD) of the spike protein fused with the tetanus toxoid epitope P2 (RBD-P2) and tested its efficacy in rodents and nonhuman primates (NHPs). We also investigated whether the SARS-CoV-2 nucleocapsid protein (N) could increase vaccine efficacy. Immunization with N and RBD-P2 (RBDP2/N) + alum increased T cell responses in mice and neutralizing antibody levels in rats compared with those obtained using RBD-P2 + alum. Furthermore, in NHPs, RBD-P2/N + alum induced slightly faster SARS-CoV-2 clearance than that induced by RBD-P2 + alum, albeit without statistical significance. Our study supports further development of RBD-P2 as a vaccine candidate against SARS-CoV-2. Also, it provides insights regarding the use of N in protein-based vaccines against SARS-CoV-2.

Nanoformulated Single-Stranded RNA-Based Adjuvant with a Coordinative Amphiphile as an Effective Stabilizer: Inducing Humoral Immune Response by Activation of Antigen-Presenting Cells

As agonists of TLR7/8, single-stranded RNAs (ssRNAs) are safe and promising adjuvants that do not cause off-target effects or innate immune overactivation. However, low stability prevents them from mounting sufficient immune responses. This study evaluates the adjuvant effects of ssRNA derived from the cricket paralysis virus intergenic region internal ribosome entry site, formulated as nanoparticles with a coordinative amphiphile, containing a zinc/dipicolylamine complex moiety as a coordinative phosphate binder, as a stabilizer for RNA-based adjuvants. The nanoformulated ssRNA adjuvant was resistant to enzymatic degradation in vitro and in vivo, and that with a coordinative amphiphile bearing an oleyl group (CA-O) was approximately 100 nm, promoted effective recognition, and improved activation of antigen-presenting cells, leading to better induction of neutralizing antibodies following single immunization. Hence, CA-O may increase the efficacy of ssRNA-based adjuvants, proving useful to meet the urgent need for vaccines during pathogen outbreaks.

Cricket paralysis virus internal ribosome entry site-derived RNA promotes conventional vaccine efficacy by enhancing a balanced Th1/Th2 response

•

RNA adjuvant was developed from the CrPV intergenic region IRES.

•

The RNA adjuvant functioned as an adjuvant with protein-based vaccines.

•

The RNA adjuvant increased vaccine efficacy and induced balanced Th1/Th2 response.

•

The RNA adjuvant enhanced APC chemotaxis.

An ideal adjuvant should increase vaccine efficacy through balanced Th1/Th2 responses and be safe to use. Recombinant protein-based vaccines are usually formulated with aluminum (alum)-based adjuvants to ensure an adequate immune response. However, use of alum triggers a Th2-biased immune induction, and hence is not optimal. Although the adjuvanticity of RNA has been reported, a systematic and overall investigation on its efficacy is lacking. We found that single strand RNA (termed RNA adjuvant) derived from cricket paralysis virus intergenic region internal ribosome entry site induced the expression of various adjuvant-function-related genes, such as type 1 and 2 interferon (IFN) and toll-like receptor (TLR), T cell activation, and leukocyte chemotaxis in human peripheral blood mononuclear cells; furthermore, its innate and IFN transcriptome profile patterns were similar to those of a live-attenuated yellow fever vaccine. This suggests that protein-based vaccines formulated using RNA adjuvant function as live-attenuated vaccines. Application of the RNA adjuvant in mouse enhanced the efficacy of Middle East respiratory syndrome spike protein, a protein-subunit vaccine and human papillomavirus L1 protein, a virus-like particle vaccine, by activating innate immune response through TLR7 and enhancing pAPC chemotaxis, leading to a balanced Th1/Th2 responses. Moreover, the combination of alum and the RNA adjuvant synergistically induced humoral and cellular immune responses and endowed long-term immunity. Therefore, RNA adjuvants have broad applicability and can be used with all conventional vaccines to improve vaccine efficacy qualitatively and quantitively.

Evaluation of the safety profile of ssRNA adjuvant derived from the IGR IRES region of CrPV

Adjuvants enhance the efficacy of vaccines by stimulating immune response relate gene expression and pathways. However, development of an appropriate adjuvant for a specific vaccine type is difficult, because of their unwanted systemic and local adverse effects. Despite intensive efforts for more than 80 years, there are only a few approved adjuvants. Poly I:C, a double stranded RNA was experimentally used as an adjuvant since 1970s. Although poly I:C dramatically stimulates the innate immune response, which is a prerequisite of a good adjuvant, inducing effective immune response while avoiding adverse events is a major challenge. In this study, we evaluated the safety and toxicological profile of a high-dose (200ug/mouse) single stranded RNA (ssRNA) derived from the intergenic region (IGR) internal ribosome entry site (IRES) of cricket paralysis virus (CrPV) as a novel adjuvant in a mouse model. The results indicate that there were no significant differences in body weight and food intake pattern in mice immunized with ssRNA adjuvant and the control group. Hematological parameters like ALT and AST levels appeared to be normal in both treated and control group. Histology of organs such as liver, spleen, lung, and kidney appeared to be normal. There was no evidence of tissue damage in mice treated with ssRNA adjuvant and control group. IgE levels, which is marker of autoimmune disease, is also not different compared with treated group and control group. In addition, the adjuvant effect is similar in both high-dose and low-dose (20ug/mouse) treated groups. In conclusion, novel ssRNA adjuvant exhibited a safe toxicological profile in mouse model. The results obtained may be utilized for conducting further clinical trials in humans.

Mid-long term effect of non-ablative high radiofrequency therapy on the rabbit dermal extracellular matrix

This study quantitatively investigated the postoperative effects of radiofrequency (RF) application on the normal dermal extracellular matrix (ECM) of in vivo rabbits. Postoperative effects were evaluated by histology and atomic force microscopy analysis of dermal tissues treated using three RF energy levels (10 ~ 30 W) and either a single- or multiple-pass procedure. Progressive changes in the morphology of rabbit dermal ECMs were investigated over a 30-day postoperartive period. All RF-treated groups, except for the low energy group (10 W), displayed more prominent inflammatory responses compared to the control. This inflammatory reaction was more prominent a day after application. Dermal tissues 30 days after RF application exhibited prominent myofibroblast activity associated with ECM contractile activity during wound healing in addition to chronic inflammation. A decrease in the morphology of dermal ECMs after RF application continued until seven days postoperatively. The ECM diameter increased to near baseline at 30 days postoperatively. Low energy and multi-pass applications resulted in greater collagen fibril contraction and recovery at the ultra-structural level at 30 days postoperatively than did a single high energy application.

Determination of dexamethasone sodium phosphate in the vitreous by high performance liquid chromatography

Dexamethasone sodium phosphate (DSP) is increasingly used in the treatment of ocular inflammatory diseases by systemic, periocular, and recently, intravitreal injection. We have developed a method for the determination of vitreous levels of DSP by reverse phase HPLC. In this method, co-elution of vitreous proteins with DSP is resolved by a prior sample clean-up procedure using Waters Sep-Pak C18 cartridge; the protein is separated and eluted with water while DSP, paraben and prednisone are eluted with methanol. DSP in the resulting sample is then separated by reverse phase HPLC and quantified by UV absorption at 254 nm. The recovery of DSP through the sample clean-up is 68.9 +/- 3.0%. DSP quantitation is linear from 0.1 mg to 1.0 mg per 1.0 ml vitreous. This method provides a simple, sensitive and reliable technique for determining the vitreous levels of DSP.

Time courses in the negative and positive repetition effects

Time courses of the negative repetition effect (NRE), a poorer detection of the target in noise-same-as-target than in noise-alternative-target displays, and its opposite, the positive repetition effect (PRE), were examined. Experiment 1 showed that displays in which a low-contrast target was present with a high-contrast noise produced a larger NRE than did displays in which a contrast relationship between items was reversed. A negative contrast repetition effect (NCRE) was also found that was comparable to the NRE. Experiment 2 showed that dimensional Korean letters (e.g., [symbol: see text]) whose configural differences were apparent in orientation produced the largest PRE at a 0-ms stimulus onset asynchrony, whereas featural Korean letters (e.g., [symbol: see text]) that differed in the number of elements yielded the largest NRE when a noise letter preceded a target letter by 50 ms. Experiments 3A and 3B indicated that the NCRE may arise from spatial attention.

Consequences of allocating attention to locations and to other attributes

Simple reaction time to a light target may be lengthened when the light is preceded by a noninformative stimulus at the same location. This is known as inhibition of return. Does inhibition of return result if the relation between successive stimuli is defined in terms of color or orientation? Subjects pressed a key when a small square was displayed. In Experiments 1-3, location and color of the square were manipulated; there was inhibition of return based on location, but not on color. In Experiments 4-5, a confounding of color and luminance was eliminated, with no change in results. In Experiments 6-7, the background was composed of vertical stripes and the squares were composed of left- or right-oriented diagonal stripes. There was evidence of inhibition of return based on location, but not on orientation. These data support the idea that location is processed differently from other features.

Evaluation of the retinal toxicity and pharmacokinetics of dexamethasone after intravitreal injection

The intravitreal injection of steroids may be potentially useful in the treatment of endophthalmitis and other ocular inflammatory diseases. The retinal toxicity and intraocular turnover of aqueous solutions of dexamethasone sodium phosphate in doses ranging from 440 to 4000 micrograms were evaluated in the rabbit; evaluation was also performed for a 0.1-mL injection of a commercially available preparation (dexamethasone phosphate [Decadron] injection, 4 mg/mL). After the 440-micrograms dose, a transient increase in staining of the Müller cells was observed, which normalized after 2 days. Progressively higher doses resulted in an increasing spectrum of disorganization in Müller and other retinal cells. The half-life of the intravitreally injected drug was 3.48 hours. These findings suggest a primary interference in Müller cell function, possibly through dexamethasone-induced alterations in retinal glutamate or glucose metabolism.

Intravitreal antibiotic therapy with vancomycin and aminoglycoside: examination of the retinal toxicity of repetitive injections after vitreous and lens surgery

The combination of vancomycin and an aminoglycoside antibiotic is frequently injected into aphakic/vitrectomized eyes for the treatment of endophthalmitis. At 2 weeks after lens and vitreous removal, rabbit eyes received an injection of a combination of 1 mg vancomycin and either 400 micrograms amikacin or 100 micrograms gentamicin. Furthermore, the effects of repeating the same combination a second and third time at intervals of 48 h were examined by light and transmission electron microscopy at 7 days to 4 months after the last injection. A single injection of either combination produced no toxicity. After 2 or 3 sets of injections of either combination, 13 of 16 eyes displayed retinal toxicity manifested by macrophages in the subretinal space, disorganization of the outer segments and retinal pigment epithelium, and discontinuities in Bruch's membrane. These results indicate that whereas a single injection of these combinations is not toxic to aphakic/vitrectomized eyes, repetitive injections may result in increasing toxicity.

Further evidence for a time-independent shift of the focus of attention

The separation between stimuli was manipulated in a same-different matching task. In Experiment 1, stimuli were upright Ts and/or Ls, whereas in Experiment 2, they were rotated Ts and/or Ls. In both experiments, mean reaction time (RT) for the same-different judgment did not increase as a function of interletter separation, suggesting either that the time needed to relocate attention was independent of distance, or that the stimuli were processed in parallel. These alternatives were tested in a third experiment, with a diagnostic for parallel processing proposed by Egeth and Dagenbach (1991). The diagnostic indicated that the rotated Ts and Ls in Experiment 2 were processed serially. If serial processing implies the utilization of attention, then the results of Experiment 2 suggest that relocation of attention is time-invariant with respect to distance.

Stimulus repetition effects and the dimension-feature distinction in alternative targets

The negative repetition effect (NRE) refers to a poorer perception of the target in noise-same-as-target than in noise-alternative-target displays. Using the parenthesis as target and noise that are either identical or different in orientation, we tested predictions derived from a feature-specific inhibition model (Bjork & Murray, 1977) that explains the NRE as arising from inhibitory interactions among channels handling identical inputs. Contrary to our expectation of evidence for orientation-specific inhibition, a positive repetition effect (PRE) was observed. We also found a PRE for the dimension target sets whose stimuli differ only in orientation and an NRE for the feature target sets whose stimuli differ in the number of elements. Further experiments that systematically varied relationships between alternative targets indicate that these basic findings, termed the stimulus attributes effects, do not arise from pattern masking, perceptual strategies, or response bias.

Stimulus repetition effects and the dimension-feature distinction in alternative targets

The negative repetition effect (NRE) refers to a poorer perception of the target in noise-same-as-target than in noise-alternative-target displays. Using the parenthesis as target and noise that are either identical or different in orientation, we tested predictions derived from a feature-specific inhibition model (Bjork & Murray, 1977) that explains the NRE as arising from inhibitory interactions among channels handling identical inputs. Contrary to our expectation of evidence for orientation-specific inhibition, a positive repetition effect (PRE) was observed. We also found a PRE for the dimension target sets whose stimuli differ only in orientation and an NRE for the feature target sets whose stimuli differ in the number of elements. Further experiments that systematically varied relationships between alternative targets indicate that these basic findings, termed the stimulus attributes effects, do not arise from pattern masking, perceptual strategies, or response bias.