A109 TRANEXAMIC ACID TO PREVENT BLEEDING AFTER ENDOSCOPIC RESECTION OF LARGE COLORECTAL POLYPS: A PILOT PROJECT

Background

Colonoscopy and polypectomy reduce colorectal cancer incidence and mortality, but is also associated with adverse events, including bleeding. Postpolypectomy delayed bleeding (PPDB) after EMR of large colorectal polyps (≥2cm) has an incidence of 2.6-9.7%. Tranexamic acid is a member of a class of drugs called antifibrinolytic agents. It reduces fibrinolysis by slowing down the conversion of plasminogen to plasmin, which may prevent bleeding.

Purpose

The goal of this pilot study is to assess the feasibility of using tranexamic acid after EMR of large (≥2 cm) non-pedunculated colorectal polyps (LNPCPs) to prevent PPDB.

Method

This was a single center feasibility study conducted at the Kingston Health Sciences Center from March 2021 to September 2021. Patients referred for removal of a ≥2cm LNPCP and those who were referred for a positive fecal immunochemical test were approached for consideration of inclusion. Patients with INR ≥ 1.5, platelets <50, higher risk of risk of thromboembolic events (atrial fibrillation on anticoagulation, history of stroke, TIA, pulmonary embolism, deep vein thrombosis hypercoagulable state, mechanical heart valve on anticoagulation, myocardial infarction in the last twelve months), pregnancy or undergoing ESD were not included.

Coagulation of submucosal vessels after polypectomy by snare tip coagulation or forceps was performed if thought necessary by the endoscopist. Clipping could be performed only where there was concern for perforation. Intraprocedural bleeding was recorded and managed at the discretion of the endoscopist.

After the procedure was completed, 1 gram of TXA in 100mL of normal saline (NS) was infused over a 10-minute interval. The participants received tranexamic acid 1 gram PO TID to be taken for 5 days after the procedure. A post procedure day 5, 14 and 30 phone call was conducted with participants to monitor study drug compliance and adverse events.

Result(s)

A total of 25 patients were enrolled with a mean polyp size of 3 cm. Baseline patient and polyp characteristics are presented in table 1. 90% of eligible patients approached consented to be in the study. Procedure details are presented in table 2. Intraprocedural bleeding occurred in 7 patients (28%) and all of these were treated with soft coagulation. 2 patients had clipping for muscle injury. All 25 patients received IV TXA post procedure. 16 patients (64%) took every dose of the prescribed pills. 21 patients (84%) took at least 80% of the prescribed TXA pills. 1 patient presented with post polypectomy bleeding. All patients completed the day 30 follow up phone call. There were no adverse events.

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Conclusion(s)

TXA to prevent postpolypectomy delayed bleeding (PPDB) was feasible to use with no adverse events reported. All patients received IV TXA post procedure and completed 30 day follow up. However, only 64% of patients took every scheduled dose of medication. A randomized controlled study will be needed to see if TXA can significantly reduce PPDB.

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Other

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Queen's University DOM Research Award – Clinical Innovation

Disclosure of Interest

None Declared

A280 DOES THE PLACEMENT OF A MANOMETRY PROBE WITH ENDOSCOPY AND CONSCIOUS SEDATION EFFECT MEASUREMENTS ON ENSUING HIGH RESOLUTION MANOMETRY? A RETROSPECTIVE COHORT STUDY IN PATIENTS POST-PERORAL ENDOSCOPIC MYOTOMY

Background

High resolution manometry (HRM) is the gold standard for diagnosis of esophageal motility disorders. This is traditionally conducted without sedation, using intranasal intubation with a manometry probe. Inserting the HRM probe immediately after endoscopic examination allows for endoscopic visualization and sedation for probe placement. This method is used commonly used for our patients who are post-peroral endoscopic myotomy (POEM). However, it is unclear whether short acting sedative drugs administered during endoscopy can affect contractility measurements during HRM.

Purpose

The purpose of this study was to determine if conscious sedation with short-acting anaesthetic agents (Midazolam and Fentanyl) influenced HRM readings in patients undergoing evaluation post-perioral endoscopic myotomy (POEM).

Method

We conducted a retrospective cohort study, comparing patients post-POEM who had endoscopy under conscious sedation with placement of the manometry probe versus those without preceeding endoscopy/sedation. Post-POEM patients who had undergone HRM over a 3-year period (2019-2022) were identified from motility lab records. Electronic charts were reviewed for data abstraction, and HRM parameters (resting lower esophageal pressure and integrated relaxation pressure) were determined for both groups and compared using t-tests, and error listed as standard error of the mean.

Result(s)

From 2019-2022, 15 patients underwent manometry using a non-endoscopic approach with no sedation and 25 underwent manometry following endoscopy with conscious sedation and placement of the probe during the procedure. In the latter group, HRM occurred within 2 hours of probe placement. The mean doses of fentanyl and midazolam were 115mcg (+/-29.7) and 3.3mg (+/-1.3) respectively. The mean end-expiratory lower esophageal sphincter pressure was 9.8 mmHg (+/- 2.9) in the non-endoscopic group and 26.24mmHg (+/- 5) in the endoscopy group (p <0.02). The median integrated relaxation pressure was 11.2 mmHg (+/-1.2) in the non-endoscopic group and 14.52mmHg (+/- 0.9) in those in the endoscopy group (p <0.04).

Conclusion(s)

In conclusion, this small, retrospective cohort study shows that manometry probe placement after endoscopy with conscious sedation may affect esophgeal HRM measurement parameters in the post-POEM patient population. Physicians should consider delaying HRM measurements for >4 hours to maximize clearance of the sedative medications.

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None

Disclosure of Interest

None Declared

A114 UNIFIED MAGNIFYING ENDOSCOPIC CLASSIFICATION (UMEC) FOR GASTROINTESTINAL LESIONS: A NORTH AMERICAN EDUCATION STUDY

Background

Magnification endoscopy and magnification narrow-band imaging are image enhanced endoscopy technologies that may allow for the diagnosis of advanced neoplasia in the GI tract on the basis of imaging characteristics. Recently, the Unified Magnifying Endoscopic Classification (UMEC) has been developed, which unified the criteria for the esophagus, stomach, and colon. UMEC divides optical diagnosis into one of the three categories: non-neoplastic, intramucosal neoplasia, and deep submucosal invasive cancer.

Purpose

The objective of this study is to educate North American endoscopists on the use of the UMEC schema, and to ascertain performance of the UMEC framework among North American endoscopists.

Method

Using UMEC, five North American endoscopists (&gt;1000 procedures) without prior training in magnifying endoscopy independently diagnosed previously collected endoscopic image set of the esophagus, stomach, and colon. The endoscopists were trained on the use of UMEC via an eleven-minute training video with exemplars of each element of UMEC from esophagus, stomach, and colon. All endoscopists were blinded to white-light and non-magnifying NBI findings as well as histopathological diagnosis. The diagnostic performance of UMEC was assessed while using the gold standard histopathology as a reference.

Result(s)

A total of 299 gastrointestinal lesions (77 esophagus, 92 stomach, and 130 colon) were assessed using UMEC. For esophageal squamous cell carcinoma, the sensitivity, specificity, and accuracy for all 5 endoscopists ranged from 65.2% (95% CI: 50.9–77.9) to 87.0% (95% CI: 75.3–94.6), 77.4% (95% CI: 60.9–89.6) to 96.8% (95% CI: 86.8–99.8), and 75.3% to 87.0%, respectively. For gastric adenocarcinoma, the sensitivity, specificity, and accuracy for all 5 endoscopists ranged from 94.9% (95% CI: 85.0–99.1) to 100%, 52.9% (95% CI: 39.4–66.2) to 92.2% (95% CI: 82.7–97.5), and 73.3% to 93.3%, respectively. For colorectal adenocarcinoma, the sensitivity, specificity, and accuracy for all 5 endoscopists ranged from 76.2% (95% CI: 62.0–87.3) to 83.3% (95% CI: 70.3–92.5), 89.7% (95% CI: 82.1–94.9) to 97.7% (95% CI: 93.1–99.6), and 86.8% to 90.7%, respectively.

Image

Conclusion(s)

UMEC is a simple and practical classification that can be used to introduce and educate endoscopists to magnification narrow-band imaging and optical diagnosis.

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CAG

Disclosure of Interest

M. R. A. Fujiyoshi Grant / Research support from: 2022 CAG/AbbVie Education Research Grant, Y. Fujiyoshi: None Declared, N. Gimpaya: None Declared, R. Bechara: None Declared, T. Jeyalingam: None Declared, N. Calo: None Declared, N. Forbes: None Declared, R. Khan: None Declared, M. Atalla: None Declared, A. Toshimori: None Declared, Y. Shimamura: None Declared, M. Tanabe: None Declared, J. Mosko: None Declared, H. Inoue: None Declared, S. Grover: None Declared

A138 ENDOSCOPIC SUBMUCOSAL DISSECTION FOR BARRETT’S NEOPLASIA IN A NORTH AMERICAN ACADEMIC CENTRE

Background

Endoscopic submucosal dissection (ESD) is carving out an increasing role in the treatment of Barrett’s associated neoplastic lesions. Though endoscopic therapy is classically performed with endoscopic mucosal resection (EMR) and radiofrequency ablation (RFA), ESD provides the advantage of en-bloc resections, and greater R0 resection rates, which are defined as negative deep and lateral margins on histology.

Aims

Our aim is to present procedural outcomes and subgroup analysis from one of the largest single centre cohorts of esophageal ESD in North America, and the first from Canada.

Methods

All patients undergoing esophageal ESD for Barrett’s neoplasia between Oct 2016 and June 2020 at a single tertiary care centre in Canada were included in the cohort. Demographic, procedural data and lesion characteristics are presented. Subgroup analysis was performed on patients who underwent extensive resection (≥75% of esophageal circumference) and patients who developed strictures. Statistical analysis included chi square testing on categorical variables and unpaired t-test for continuous variables. Binomial univariable logistic regression was performed to investigate factors associated with stricture formation.

Results

Thirty-four patients underwent esophageal ESD for Barrett’s associated neoplasia during the study period. The median lesion diameter was 5.7 cm (IQR 4.2 -7.5) and median procedure time 129 min (IQR 66–200). The en-bloc resection rate was 97%, and the R0 resection rate was 91%. Curative resection was achieved in 82% of patients. Upstaging from the pre-resection biopsy to post-ESD histology occurred in 59% of cases. Two adverse events occurred (1 delayed bleed, 1 aspiration event). There were no perforations. Procedural outcomes were similar between patients with extensive resections, but patients with ≥75% circumferential resection developed more strictures (65% vs 13%, p=0.001). Stricture formation was associated with extensive resection (OR 15.4, 95%CI[2.50 - 95.01]) and longer lesion diameter (OR 1.73, 95%CI [1.11 - 2.70]).

Conclusions

Our experience with ESD for Barrett’s related neoplasia shows excellent en-bloc and R0 resection rate, and provides more accurate histological specimens. Curative resection is possible in the vast majority of cases, including those with extensive resections. Further investigation into stricture prophylaxis will be useful as larger resections are attempted.

Funding Agencies

None

A195 THE CANADIAN POEM EXPERIENCE. THE FIRST 50 PATIENTS

Background

Achalasia is a disorder of esophageal motility characterized by a loss of enteric neurons resulting in impaired relaxation of the lower esophageal sphincter and absence of esophageal peristalsis. There are no current treatments that allow for regeneration of the enteric neurons. Interventions focus primarily on lowering the LES pressure to provide symptom relief and improve quality of life. Peroral endoscopic myotomy (POEM) has emerged as a less invasive technique for performing myotomy in patients with achalasia.

Aims

This study aims to assess the safety and efficacy of POEM in a Canadian tertiary care center.

Methods

All consecutive patients who underwent POEM between March 2016 and May 2018 at a tertiary center were included. The primary outcome of the study was clinical success rate of POEM defined as a post-POEM Eckardt score ≤ 3 at ≥ 3 months. Adverse events were recorded according to the Clavien-Dindo grading system.

Results

A total of 50 consecutive patients underwent 51 POEM procedures with a mean procedure length of 85.6±29.6 minutes. Post-POEM Eckardt scores of ≤ 3 at ≥ 3 months was achieved in 98% of patients. The incidence of pathologic reflux post-poem was 23%. The median length of hospital stay was 1 day (range 1–2 days). No major adverse events occurred.

Conclusions

POEM is a safe and effective procedure for the treatment of achalasia. At a median follow up of 19.5 months, 98% of patients had sustained clinical response (Eckardt score ≤ 3).

Funding Agencies

None

A132 SMOOTH MUSCLE BIOPSIES FROM POEM PROCEDURES FOR ACHALASIA SHOW LACK OF FUNCTIONAL INNERVATION

Background

Idiopathic achalasia is a disease of the esophagus causing impaired peristalsis and absent lower esophageal sphincter relaxation. The etiopathogenesis of the disease is unclear but most histopathologic studies from surgical resections show an absence of nitric oxide-producing neurons in the myenteric plexus. The peroral endoscopic myotomy (POEM) procedure has evolved in the last decade to treat achalasia and has provided a unique way to sample diseased tissue from an otherwise inaccessible tissue compartment.

Aims

To study the effects of achalasia on innervation of smooth muscle and smooth muscle phenotype.

Methods

Patients with a diagnosis of achalasia based on high resolution manometry undergoing a POEM at Kingston general hospital were approached for enrollment between June 2017 to September 2018. Demographic information including age, symptom duration, previous treatments, and Eckhardt score was collected. Intraoperatively, biopsies of the circular smooth muscle (CSM) layer were taken at the proximal and distal extents of the myotomy and placed in formalin. Tissue was embedded in wax, sectioned and stained using immunocytochemistry (ICC) for neuronal, axonal and smooth muscle cell markers. This was compared to control tissue from patients undergoing gastroesophagectomy for adjacent malignancy.

Results

Control tissue from 3 separate esophagectomies were obtained. Samples were obtained from a total of 25 patients (13 males), with a median age of 50 (IQR 38–67). Most patients had Type 2 achalasia (19 [76%]) followed by Type 1 and 3 (2, [8%] respectively). Two cases had conflicting manometric findings. The median duration of symptoms was 3 years (IQR 1.5–10.5) and the median Eckhardt score was 6.5 (IQR 5–9). In sample tissues, no neuronal cell bodies were detected in the CSM layer. ICC for the axonal marker PGP9.5 showed that CSM of achalasia samples were almost completely devoid of axon structure, independent of the subtype of achalasia, compared to abundant axon presence in control tissues. In parallel, ICC showed that cholinergic (ChAT) or nitrergic (nNOS) axonal subtypes were absent in biopsy CSM while abundant in controls. CSM cells displayed hypertrophy with no detection of proliferation by ICC (KI67) or alterations in the phenotypic marker SM-22.

Conclusions

Preliminary results from advanced immunohistochemical techniques show the absence of functional innervation of the CSM layer of all patients with achalasia. This is characterized by a depleted excitatory and inhibitory axon population. Further studies are focused on defining differences in smooth muscle phenotype and the presence or absence of inflammatory cells within the CSM.

Funding Agencies

None

A158 LACK OF DIFFERENCE OF COLONIC CURVATURE IN SUPINE VERSUS PRONE PATIENT POSITIONS, IN NORMAL AND HIGH BMI INDIVIDUALS, AS ASSESSED BY QUANTITATIVE ASSESSMENT OF COMPUTED TOMOGRAPHY COLONOGRAPHY

Background

Dynamic positional changes during colonoscopy are commonly used in clinical practice, in particular moving from side to side. It has been shown to improve both adenoma detection rates as well as cecal intubation times. However, perhaps due to an additional level of inconvenience, there have been few studies comparing the anatomy and changes in colonic curvature when patients are in the prone position, which may help to prevent anterior bowing of the scope, particularly in patients with high body mass index (BMI).

Aims

To compare both the number of colonic curves and degree of change in curves with patients in supine versus prone positioning during computed tomography colonography (CTC).

Methods

75 CTC studies, obtained between January and April 2017 at Hotel Dieu Hospital in Kingston, Ontario, were screened and included based on image quality and adequacy of distention. Per standardized protocol, all patients undergoing CTC are imaged both in supine and prone positioning. Using an automated computer algorithm process developed for this study, curves were identified and measured via centerline points placed digitally through the colonic lumen, and compared between supine and prone patient positioning.

Results

75 colonographies were examined. The mean age was 68 years and 37/75 were male. BMI data was available for 56 patients, with mean BMI 29.4 (SD 5.7). There were no significant differences in total mean degrees of curvature between supine and prone positions [75.3 (SD 13.5) vs. 77.3 (SD 15.3), p=0.07], nor a significantly higher total number of curves &gt;100 degrees [4.0 (SD 2.0) vs. 4.5 (SD 2.3), p=0.14]. No significant correlation was seen between BMI and change in position (correlation factor 0.2, p=0.13).

Conclusions

No significant differences were found between the two positions during CT colonography. This certainly calls into question the strategy of starting in prone position, even in higher BMI patients. However, CT colonography doesn’t account for changes that can occur during colonoscopy, as the scope itself can dynamically affect angulations within the colon.

Funding Agencies

None

A270 AMPULLARY MORPHOLOGY PREDICTS SUCCESS AND POST-PROCEDURE COMPLICATIONS IN PRIMARY NEEDLE-KNIFE FISTULOTOMY FOR BILIARY CANNULATION IN ERCP: RESULTS OF A PROSPECTIVE CASE SERIES

Background

Biliary cannulation is often the most challenging component of successful ERCP. Needle-knife fistulotomy (NKF) involves incising directly into the intra-duodenal portion of the bile duct, away from the native orifice, to achieve direct biliary access. This technique is usually performed when standard cannulation is unsuccessful, however recently, has been shown by our group to be a potentially effective and safe method to achieve biliary access as the primary approach. It has also been demonstrated that ampullary morphology influences cannulation success.

Aims

This study aims to assess the impact of ampullary morphology on the success and complications of NKF.

Methods

A retrospective visual analysis was performed of the ampullary morphology collected prospectively as part of our original NKF study (NCT03698266). Endoscopic videos and photos were reviewed to classify the ampulla into one of four types, as per a validated classification system. Ampullas were either Type 1 (regular), Type 2 (small), Type 3 (protruding or pendulous) or Type 4 (creased or ridged). Outcomes examined included: time to cannulation, proceduralist-perceived difficulty and complications.

Results

Of the 71 patients enrolled in the NKF study, 66 had video or photographs of the ampulla to facilitate classification. There were 35 (52%) Type 1 ampullas, 16 (24%) Type 2 and 14 (21%) Type 3 and one (1%) Type 4 ampullas. 65 out of the 66 (98.4%) patients had successful biliary access after primary NKF. The mean time to cannulation in all patients was 5.3 minutes (range 0.25–27 minutes). There were 3 cases of post-ERCP pancreatitis (PEP) (4.5%) and 3 cases of post-ERCP bleeding (4.5%).

Compared to Type 1 ampullas, there was a no significant increase in time to cannulation in Type 2 and Type 3 ampullas using NKF (Type 1 4.7 vs. Type 2 7.1 vs. Type 3 6.4 minutes, p = NS). Only one patient with a Type 2 ampulla was rated as ‘low difficulty’ by the proceduralist performing NKF. Nevertheless, successful NKF was achieved in 15 out of 16 Type 2 ampullas (93.8%). There was 100% success rate in biliary access for Type 3 ampullas, previously shown to more challenging using conventional cannulation techniques. All cases of PEP occurred in patients with Type 2 ampullas.

Conclusions

As with the impact on conventional cannulation, ampullary morphology may influence outcomes of NKF. It appears that small, Type 2 ampullas with a short intra-duodenal segment makes NKF more challenging and may be associated with a higher rate of PEP. Unlike conventional cannulation, Type 3 ampullas are similar to Type 1 ampullas in ease of NKF access.

Funding Agencies

None

Identification of T-cell epitopes from benzylpenicillin conjugated to human serum albumin and implication in penicillin allergy

BACKGROUND

There is in vitro evidence that T cells from allergic patients react to benzylpenicillin-human serum albumin (BP-HSA) bioconjugates. Our group has recently shown the existence of naïve CD4⁺ T cells recognizing BP-HSA in healthy donors. However, BP-haptenated peptides from HSA participating in the immunization of allergic patients have never been identified. The purpose of the present study is to identify immunodominant BP-haptenated peptides from HSA involved in immunization of patients to BP and to refine the frequency calculation of naïve CD4⁺ T cells recognizing BP.

METHODS

Co-cultures were established with CD4⁺ T cells from non-allergic donors and mature autologous dendritic cells (DCs) loaded with BP-HSA or BP-haptenated peptides from HSA. The CD4⁺ T-cell response specific for BP-HSA or for individual BP-haptenated peptides was measured using an interferon-γ (IFN-γ) ELISpot assay. The frequency of BP-specific CD4⁺ T cells was then calculated using the Poisson distribution. BP-HSA and BP-haptenated peptides recognition by allergic patients was evaluated on peripheral blood mononuclear cells (PBMCs) using a lymphocyte transformation test (LTT).

RESULTS

Results showed that BP-HSA and BP-haptenated peptides were recognized by naïve T cells from 15/16 and 13/14 tested healthy donors, respectively. Most donors responded to 3 peptides with BP covalently bound on lysines 159, 212, and 525. Two of these benzylpenicilloylated peptides (lysines 159 and 525) were also found to induce PBMCs proliferation in patients with allergic reaction to penicillins.

CONCLUSION

This study identifies and characterizes for the first time the BP-haptenated peptides from HSA involved in the immunization of patients to penicillins.

Monoclonal, but not polyclonal, antibodies protect against Plasmodium yoelii sporozoites

One of the primary strategies for malaria vaccine development has been to design subunit vaccines that induce protective levels of antibodies against the circumsporozoite (CS) protein of malaria sporozoites. In the Plasmodium yoelii mouse model system such vaccines have been uniformly unsuccessful in protecting against sporozoite-induced malaria. To demonstrate that antibodies to P. yoelii CS protein could provide protection we established a passive transfer model. Passive transfer of Navy yoelii sporozoite 1 (NYS1), an IgG3 mAb against the P. yoelii CS protein, protected 100% of mice against challenge with 5000 P. yoelii sporozoites. Binding of NYS1 to sporozoites was inhibited by incubation with (QGPGAP)2, a synthetic peptide derived from the repeat region of the P. yoelii CS protein, indicating that the epitope on sporozoites recognized by this mAb was included within this peptide. The levels of antibodies to (QGPGAP)2 by ELISA, and to sporozoites by indirect fluorescent antibody test and CS precipitation reaction were similar in sera from mice that received NYS1 in passive transfer and were protected against challenge with 5000 sporozoites, and from mice that had been immunized with subunit vaccines containing (QGPGAP)2 but were not protected against challenge with 40-200 sporozoites. To determine if antibody avidity, not absolute concentration could explain the striking differences in protection, we established a thiocyanate elution assay. The results suggest that NYS1, the protective mAb, has a lower avidity for (QGPGAP)2 and for sporozoites than do the vaccine-induced antibodies. Although the results of the conventional antibody assays did not correlate with protection, sera from the protected animals inhibited sporozoite development in mouse hepatocyte cultures significantly more than did the sera from the unprotected, subunit vaccine-immunized animals, correlating with protection. The data clearly demonstrate that antibodies to the CS protein can protect against intense sporozoite infection. Improved understanding of the differences between protective mAb and nonprotective polyclonal antibodies will be important in the further development of malaria vaccines.

Monoclonal, but not polyclonal, antibodies protect against Plasmodium yoelii sporozoites

One of the primary strategies for malaria vaccine development has been to design subunit vaccines that induce protective levels of antibodies against the circumsporozoite (CS) protein of malaria sporozoites. In the Plasmodium yoelii mouse model system such vaccines have been uniformly unsuccessful in protecting against sporozoite-induced malaria. To demonstrate that antibodies to P. yoelii CS protein could provide protection we established a passive transfer model. Passive transfer of Navy yoelii sporozoite 1 (NYS1), an IgG3 mAb against the P. yoelii CS protein, protected 100% of mice against challenge with 5000 P. yoelii sporozoites. Binding of NYS1 to sporozoites was inhibited by incubation with (QGPGAP)2, a synthetic peptide derived from the repeat region of the P. yoelii CS protein, indicating that the epitope on sporozoites recognized by this mAb was included within this peptide. The levels of antibodies to (QGPGAP)2 by ELISA, and to sporozoites by indirect fluorescent antibody test and CS precipitation reaction were similar in sera from mice that received NYS1 in passive transfer and were protected against challenge with 5000 sporozoites, and from mice that had been immunized with subunit vaccines containing (QGPGAP)2 but were not protected against challenge with 40-200 sporozoites. To determine if antibody avidity, not absolute concentration could explain the striking differences in protection, we established a thiocyanate elution assay. The results suggest that NYS1, the protective mAb, has a lower avidity for (QGPGAP)2 and for sporozoites than do the vaccine-induced antibodies. Although the results of the conventional antibody assays did not correlate with protection, sera from the protected animals inhibited sporozoite development in mouse hepatocyte cultures significantly more than did the sera from the unprotected, subunit vaccine-immunized animals, correlating with protection. The data clearly demonstrate that antibodies to the CS protein can protect against intense sporozoite infection. Improved understanding of the differences between protective mAb and nonprotective polyclonal antibodies will be important in the further development of malaria vaccines.

Active and passive immunization against Plasmodium yoelii sporozoites

Three subunit vaccines based on the major repeat, (QGPGAP)n, and flanking regions of the Plasmodium yoelii circumsporozoite protein were designed, produced, and tested. All were immunogenic, but none gave consistent protection against a 40-200 sporozoite challenge. To demonstrate that antibodies to P. yoelii CS protein could provide protection we established a passive transfer model. Passive transfer of NYS1, an IgG3 MAb against the P. yoelii CS protein, protected 100% of mice against challenge with 5000 P. yoelii sporozoites. Binding of NYS1 to sporozoites was inhibited by incubation with (QGPGAP)2, indicating that the epitope on sporozoites recognized by this MAb was included within this peptide. The levels of antibodies to (QGPGAP)2 by ELISA, and to sporozoites by IFAT and CS precipitation reaction were similar in sera from mice that received NYS1 in passive transfer and were protected against challenge with 5000 sporozoites, and from mice that had been immunized with subunit vaccines containing QGPGAP but were not protected against challenge with 40-200 sporozoites. To determine if antibody avidity, not the absolute concentration, could explain the striking differences in protection, we established a thiocyanate elution assay. The results suggest that NYS1, the protective MAb, has a lower avidity for (QGPGAP)2 and for sporozoites than do the vaccine-induced antibodies. The data clearly demonstrate that antibodies to the CS protein can protect against intense sporozoite infection. Improved understanding of the differences between protective MAbs and non-protective polyclonal antibodies will be important in the further development of malaria vaccines.