Normative Data of Ocular Biometry, Optical Coherence Tomography, and Electrophysiology Conducted for Cynomolgus Macaque Monkeys

Tolerability and tropism of recombinant adeno-associated virus vectors in the African green monkey (Chlorocebus sabaeus) anterior chamber

While many studies have investigated the use of recombinant adeno-associated vectors (rAAV) in the posterior chamber for treatment of inherited retinal diseases, fewer studies have looked at rAAV's ability to transduce cells within the anterior chamber. This study focuses on evaluating the tropism and tolerability of three rAAV serotypes-rAAV2/6, rAAV2/9, and rAAV2/2[MAX]-expressing a green fluorescent protein (GFP) reporter following intracameral injection in the non-human primate (NHP) African green monkey (Chlorocebus sabaeus) model. Injection of high dose (1 × 1012 vg/eye) rAAV vector resulted in transient inflammation characterized by aqueous flare and cellular infiltrate that resolved without intervention in all serotypes. Post-mortem histology revealed widespread expression of GFP in cells of the trabecular meshwork and iris in high dose rAAV2/6, rAAV2/9, and particularly rAAV2/2[MAX] eyes, indicating that rAAV vectors of these serotypes have broad tropism for cells of the anterior chamber and may facilitate the treatment of blinding disorders, such as glaucoma.

Nonhuman Primate Eyes Display Variable Growth and Aging Rates in Alignment With Human Eyes

Purpose

To assess age-related biometric changes of the eye in nonhuman primates (NHPs), to and decipher the growth and aging rates and their comparability with humans.

Methods

Ocular anatomic measurements were performed on 341 macaca fascicularis aged 0.5 to 23 years via multimodal approaches including IOLMaster 700. Linear or polynomial regression models were simulated to determine the best fitted age-related function. The metrics were compared with human equivalents in published reports.

Results

Macaques exhibited a postnatal eye growth pattern similar to humans, characterized by continuous eye extension coordinated with dramatic reshaping of the lens but not the cornea. The age-related growth of lens thickness (LT), anterior chamber depth (ACD), and axis length (AL) exhibited nonlinear and bipolar patterns. The inflection points were 10 to 12 years old for LT and ACD and 13 to 15 years old for AL in macaques, which were comparable in chronological age at a ratio of ∼1: ratio with that in humans. In contrast, the speed of aging, including the increase in lens density and the decrease in retinal nerve fiber layer thickness, was comparable in relative age at a ratio of ∼1:3 according to the differences in lifespan between macaques and humans. Lens density was a robust indicator for the aging process.

Conclusions

Macaque eyes recapitulated the age-related process of human eyes to varying extents with different growth and aging rates. Chronological age or relative age should be considered in different scenarios when macaques are included in preclinical studies.

Correlation between spherical equivalent and biometry parameters in adult Cynomolgus macaque

PURPOSE

To characterize the distribution of refractive and ocular biometry parameters and analyze the effect factors of the refractive status in cynomolgus monkey colonies.

METHODS

A Population-based cross-sectional study was conducted in adult cynomolgus macaque colonies. Animals were anesthetized with Zoletil 50. Intraocular pressure was measured using the Icare tonometer. Cycloplegic refraction (three drops of 1% tropicamide) and corneal radius of curvature (CRC) were measured using an autorefractor. The spherical equivalent (SE) was calculated. Biometric measurements, including the anterior chamber depth (ACD), lens thickness (LT), and axial length (AL), were obtained by A-scan ultrasonography. The AL-to-CR ratio (AL/CRC) was calculated. Central corneal thickness (CCT) and choroidal thickness (ChT) were measured using the Heidelberg Spectralis HRA OCT. Multiple regression analysis was performed to explore the association between refraction and ocular biometry.

RESULTS

Among 263 cynomolgus monkeys (aged 5-26 years), which consisted of 520 eyes, 29.42% had hyperopia, 27.12% had emmetropia, 33.27% had mild-to-moderate myopia and 10.19% had high myopia. The mean SE was -1.27 ± 3.44 Diopters (D). The mean CRC, CCT, AL, and ChT was 5.70 ± 0.22 mm, 454.30 ± 32.40 μm, 18.76 ± 0.89 mm and 188.96 ± 38.19 μm, respectively. The LT was the thickest in the hyperopic eyes. CRC was the lowest, and CCT was the thickest in high myopic eyes. AL increased, while ChT decreased as SE decreased. For the SE variance, AL alone explained 40.5%; age, AL, and CRC together explained 57.5%.

CONCLUSIONS

The refractive characteristics and biometry parameters of cynomolgus monkeys are highly comparable to those of humans. AL, CRC, and ChT showed the similar variation tendency in cynomolguses when compared to humans. Cynomolgus monkeys with naturally-occurring refractive errors may be a good animal model for refractive studies.

Cynomolgus monkey's retina volume reference database based on hybrid deep learning optical coherence tomography segmentation

Cynomolgus monkeys (Macaca fascicularis) are commonly used in pre-clinical ocular studies. However, studies that report the morphological features of the macaque retina are based only on minimal sample sizes; therefore, little is known about the normal distribution and background variation. This study was conducted using optical coherence tomography (OCT) imaging to investigate the variations in retinal volumes of healthy cynomolgus monkeys and the effects of sex, origin, and eye side on the retinal volumes to establish a comprehensive reference database. A machine-learning algorithm was employed to segment the retina within the OCT data (i.e., generated pixel-wise labels). Furthermore, a classical computer vision algorithm has identified the deepest point in a foveolar depression. The retinal volumes were determined and analyzed based on this reference point and segmented retinal compartments. Notably, the overall foveolar mean volume in zone 1, which is the region of the sharpest vision, was 0.205 mm³ (range 0.154-0.268 mm³), with a relatively low coefficient of variation of 7.9%. Generally, retinal volumes exhibit a relatively low degree of variation. However, significant differences in the retinal volumes due to the monkey's origin were identified. Additionally, sex had a significant impact on the paracentral retinal volume. Therefore, the origin and sex of cynomolgus monkeys should be considered when evaluating the macaque retinal volumes based on this dataset.

Design, methodology, and preliminary results of the non-human primates eye study

PURPOSE

To describe the normative profile of ophthalmic parameters in a healthy cynomolgus monkey colony, and to identify the characteristic of the spontaneous ocular disease non-human primates (NHP) models.

METHODS

The NHP eye study was a cross-sectional on-site ocular examination with about 1,000 macaques held in Guangdong Province, southeastern China. The NHPs (Macaca fascicularis, cynomolgus) in this study included middle-aged individuals with a high prevalence of the ocular disease. The NHP eye study (NHPES) performed the information including systematic data and ocular data. Ocular examination included measurement of intraocular pressure (IOP), anterior segment- optical coherence tomography (OCT), slit-lamp examination, fundus photography, autorefraction, electroretinography, etc. Ocular diseases included measurement of refractive error, anisometropia, cataract, pterygium, etc. RESULTS: A total of 1148 subjects were included and completed the ocular examination. The average age was 16.4 ± 4.93 years. Compared to the male participants, the females in the NHPES had shorter axial length and the mean Average retinal nerve fiber layer (RNFL) thickness (except for the nasal quadrants). The mean IOP, anterior chamber depth, lens thickness, axial length, central corneal thickness, choroid thickness and other parameters were similar in each group.

CONCLUSION

The NHPES is a unique and high-quality study, this is the first large macaque monkey cohort study focusing on ocular assessment along with comprehensive evaluation. Results from the NHPES will provide important information about the normal range of ophthalmic measurements in NHP.

Outer retinal degeneration in a non-human primate model using temporary intravitreal tamponade with N-methyl-N-nitrosourea in cynomolgus monkeys

Objective:The main objective of this study was to induce and evaluate drug-dose-dependent outer retinal degeneration in cynomolgus monkeys by application of N-methyl-N-nitrosourea (MNU).Approach:Intravitreal temporary tamponade induced outer retinal degeneration with MNU solutions (2-3 mg ml^-1) after vitrectomy in five cynomolgus monkeys. Optical coherence tomography (OCT), fundus autofluorescence (FAF), full-field electroretinography (ffERG), and visual evoked potentials (VEP) were performed at baseline and weeks 2, 6, and 12 postoperatively. At week 12, OCT angiography, histology, and immunohistochemistry were performed.Main results:Outer retinal degeneration was observed in four monkeys, especially in the peripheral retina. Anatomical and functional changes occurred at week 2 and persisted until week 12. FAF images showed hypoautofluorescence dots, similar to AF patterns seen in human retinitis pigmentosa. Hyperautofluorescent lesions in the pericentral area were also observed, which corresponded to the loss of the ellipsoid zone on OCT images. OCT revealed thinning of the outer retinal layer adding to the loss of the ellipsoid zone outside the vascular arcade. Histological findings confirmed that the abovementioned changes resulted from a gradual loss of photoreceptors from the perifovea to the peripheral retina. In contrast, the inner retina, including ganglion cell layers, was preserved. Functionally, a decrease or extinction of scotopic ffERGs was observed, which indicated rod-dominant loss. Nevertheless, VEPs were relatively preserved.Significance:Therefore, we can conclude that temporary exposure to intravitreal MNU tamponade after vitrectomy induces rod-dominant outer retinal degeneration in cynomolgus monkeys, especially in the peripheral retina.

Volumetric subfield analysis of cynomolgus monkey’s choroid derived from hybrid machine learning optical coherence tomography segmentation

This study aimed to provide volumetric choroidal readings regarding sex, origin, and eye side from healthy cynomolgus monkey eyes as a reference database using optical coherence tomography (OCT) imaging. A machine learning (ML) algorithm was used to extract the choroid from the volumetric OCT data. Classical computer vision methods were then applied to automatically identify the deepest location in the foveolar depression. The choroidal thickness was determined from this reference point. A total of 374 eyes of 203 cynomolgus macaques from Asian and Mauritius origin were included in the analysis. The overall subfoveolar mean choroidal volume in zone 1, in the region of the central bouquet, was 0.156 mm³ (range, 0.131–0.193 mm³). For the central choroid volume, the coefficient of variation (CV) was found of 6.3%, indicating relatively little variation. Our results show, based on analyses of variance, that monkey origin (Asian or Mauritius) does not influence choroid volumes. Sex had a significant influence on choroidal volumes in the superior-inferior axis (p ≤ 0.01), but not in the fovea centralis. A homogeneous foveolar choroidal architecture was also observed.

Age and Sex-Related Changes in Retinal Function in the Vervet Monkey

Among the deficits in visual processing that accompany healthy aging, the earliest originate in the retina. Moreover, sex-related differences in retinal function have been increasingly recognized. To better understand the dynamics of the retinal aging trajectory, we used the light-adapted flicker electroretinogram (ERG) to functionally assess the state of the neuroretina in a large cohort of age- and sex-matched vervet monkeys (N = 35), aged 9 to 28 years old, with no signs of obvious ocular pathology. We primarily isolated the cone–bipolar axis by stimulating the retina with a standard intensity light flash (2.57 cd/s/m²) at eight different frequencies, ranging from 5 to 40 Hz. Sex-specific changes in the voltage and temporal characteristics of the flicker waveform were found in older individuals (21–28 years-old, N = 16), when compared to younger monkeys (9–20 years-old, N = 19), across all stimulus frequencies tested. Specifically, significantly prolonged implicit times were observed in older monkeys (p < 0.05), but a significant reduction of the amplitude of the response was only found in old male monkeys (p < 0.05). These changes might reflect ongoing degenerative processes targeting the retinal circuitry and the cone subsystem in particular. Altogether, our findings corroborate the existing literature in humans and other species, where aging detrimentally affects photopic retinal responses, and draw attention to the potential contribution of different hormonal environments.