Mary J. Mattapallil

The Rd8 Mutation of the Crb1 Gene Is Present in Vendor Lines of C57BL/6N Mice and Embryonic Stem Cells, and Confounds Ocular Induced Mutant Phenotypes

PURPOSE We noted an unexpected inheritance pattern of lesions in several strains of gene-manipulated mice with ocular phenotypes. The lesions, which appeared at various stages of backcross to C57BL/6, bore resemblance to the rd8 retinal degeneration phenotype. We set out to examine the prevalence of this mutation in induced mutant mouse lines, vendor C57BL/6 mice and in widely used embryonic stem cells. METHODS Ocular lesions were evaluated by fundus examination and histopathology. Detection of the rd8 mutation at the genetic level was performed by PCR with appropriate primers. Data were confirmed by DNA sequencing in selected cases. RESULTS Analysis of several induced mutant mouse lines with ocular disease phenotypes revealed that the disease was associated 100% with the presence of the rd8 mutation in the Crb1 gene rather than with the gene of interest. DNA analysis of C57BL/6 mice from common commercial vendors demonstrated the presence of the rd8 mutation in homozygous form in all C57BL/6N substrains, but not in the C57BL/6J substrain. A series of commercially available embryonic stem cells of C57BL/6N origin and C57BL/6N mouse lines used to generate ES cells also contained the rd8 mutation. Affected mice displayed ocular lesions typical of rd8, which were detectable by funduscopy and histopathology as early as 6 weeks of age. CONCLUSIONS These findings identify the presence of the rd8 mutation in the C57BL/6N mouse substrain used widely to produce transgenic and knockout mice. The results have grave implications for the vision research community who develop mouse lines to study eye disease, as presence of rd8 can produce significant disease phenotypes unrelated to the gene or genes of interest. It is suggested that researchers screen for rd8 if their mouse lines were generated on the C57BL/6N background, bear resemblance to the rd8 phenotype, or are of indeterminate origin.

An Immunologically Privileged Retinal Antigen Elicits Tolerance Major Role for Central Selection Mechanisms

Immunologically privileged retinal antigens can serve as targets of experimental autoimmune uveitis (EAU), a model for human uveitis. The tolerance status of susceptible strains, whose target antigen is not expressed in the thymus at detectable levels, is unclear. Here, we address this issue directly by analyzing the consequences of genetic deficiency versus sufficiency of a uveitogenic retinal antigen, interphotoreceptor retinoid-binding protein (IRBP). IRBP-knockout (KO) and wild-type (WT) mice on a highly EAU-susceptible background were challenged with IRBP. The KO mice had greatly elevated responses to IRBP, an altered recognition of IRBP epitopes, and their primed T cells induced exacerbated disease in WT recipients. Ultrasensitive immunohistochemical staining visualized sparse IRBP-positive cells, undetectable by conventional assays, in thymi of WT (but not of KO) mice. IRBP message was PCR amplified from these cells after microdissection. Thymus transplantation between KO and WT hosts demonstrated that this level of expression is functionally relevant and sets the threshold of immune (and autoimmune) reactivity. Namely, KO recipients of WT thymi generated reduced IRBP-specific responses, and WT recipients of KO thymi developed enhanced responses and a highly exacerbated disease. Repertoire culling and thymus-dependent CD25+ T cells were implicated in this effect. Thus, uveitis-susceptible individuals display a detectable and functionally significant tolerance to their target antigen, in which central mechanisms play a prominent role.

A humanized model of experimental autoimmune uveitis in HLA class II transgenic mice

Experimental autoimmune uveitis (EAU) is a disease of the neural retina induced by immunization with retinal antigens, such as interphotoreceptor retinoid-binding protein (IRBP) and arrestin (retinal soluble antigen, S-Ag). EAU serves as a model for human autoimmune uveitic diseases associated with major histocompatibility complex (HLA) genes, in which patients exhibit immunological responses to retinal antigens. Here we report the development of a humanized EAU model in HLA transgenic (TG) mice. HLA-DR3, -DR4, -DQ6, and -DQ8 TG mice were susceptible to IRBP-induced EAU. Importantly, HLA-DR3 TG mice developed severe EAU with S-Ag, to which wild-type mice are highly resistant. Lymphocyte proliferation was blocked by anti-HLA antibodies, confirming that antigen is functionally presented by the human MHC molecules. Disease could be transferred by immune cells with a Th1-like cytokine profile. Antigen-specific T cell repertoire, as manifested by responses to overlapping peptides derived from S-Ag or IRBP, differed from that of wild-type mice. Interestingly, DR3 TG mice, but not wild-type mice, recognized an immunodominant S-Ag epitope between residues 291 and 310 that overlaps with a region of S-Ag recognized by uveitis patients. Thus, EAU in HLA TG mice offers a new model of uveitis that should represent human disease more faithfully than currently existing models.

Mouse Models of Experimental Autoimmune Uveitis

The mouse model of experimental autoimmune uveitis, induced by immunization of mice with the retinal protein IRBP, was developed in our laboratory 20 years ago and published in 1988. Since that time it has been adopted by many investigators and has given rise to many studies that helped elucidate genetic influences, dissect the basic mechanisms of pathogenesis and test novel immunotherapeutic paradigms. The current overview will summarize the salient features of the experimental autoimmune uveitis model and discuss its mechanisms.

The Living Eye “Disarms” Uncommitted Autoreactive T Cells by Converting Them to Foxp3+ Regulatory Cells following Local Antigen Recognition

Immune privilege is used by the eye, brain, reproductive organs, and gut to preserve structural and functional integrity in the face of inflammation. The eye is arguably the most vulnerable and, therefore, also the most “privileged” of tissues; paradoxically, it remains subject to destructive autoimmunity. It has been proposed, although never proven in vivo, that the eye can induce T regulatory cells (Tregs) locally. Using Foxp3-GFP reporter mice expressing a retina-specific TCR, we now show that uncommitted T cells rapidly convert in the living eye to Foxp3+ Tregs in a process involving retinal Ag recognition, de novo Foxp3 induction, and proliferation. This takes place within the ocular tissue and is supported by retinoic acid, which is normally present in the eye because of its function in the chemistry of vision. Nonconverted T cells showed evidence of priming but appeared restricted from expressing effector function in the eye. Pre-existing ocular inflammation impeded conversion of uncommitted T cells into Tregs. Importantly, retina-specific T cells primed in vivo before introduction into the eye were resistant to Treg conversion in the ocular environment and, instead, caused severe uveitis. Thus, uncommitted T cells can be disarmed, but immune privilege is unable to protect from uveitogenic T cells that have acquired effector function prior to entering the eye. These findings shed new light on the phenomenon of immune privilege and on its role, as well as its limitations, in actively controlling immune responses in the tissue.

Increased IL-15 Production Is Associated with Higher Susceptibility of Memory CD4 T Cells to Simian Immunodeficiency Virus during Acute Infection

Acute SIV infection is characterized by explosive infection of memory CD4 T cells in peripheral and mucosal tissues. Interestingly, relatively few memory CD4 T cells are infected until as late as days 7–8 after challenge. However, by day 10 postinfection, most of the memory CD4 T cells are infected and carry viral DNA. The rapidity with which infection expands within 2–3 days to encompass virtually the entire memory CD4 T cell compartment suggests significant alterations in the susceptibility of memory CD4 T cells to infection during this period. The mechanism(s) underlying this increased permissiveness to infection is not known. In this study, we show that IL-15 secretion significantly correlates with the up-regulated expression of CD4 on memory CD4 T cells that is associated with increased permissiveness to SIV infection. Activation and proliferation of memory CD8, but not memory CD4 T cells, preceded the amplification of viral infection. Although memory CD4 T cells did not express normal activation markers, they displayed a significant up-regulation in the density of CD4 but not CCR5 expression between days 7 and 10 postinfection that correlated with increased plasma IL-15 levels and infection in these cells. Culture of purified CD4 T cells with IL-15 and/or SIV was associated with a significant increase in the expression of CD4 and infection of these sorted cells. Our results demonstrate that IL-15 contributes to the increased susceptibility of memory CD4 T cells to SIV during the early phase of acute SIV infection.

Repertoire Analysis and New Pathogenic Epitopes of IRBP in C57BL/6 (H-2b) and B10.RIII (H-2r) Mice

PURPOSE Interphotoreceptor retinoid binding protein (IRBP) is the major uveitogenic retinal antigen eliciting experimental autoimmune uveoretinitis (EAU) in mice. The most frequently used mouse strains are B10.RIII and C57BL/6, but to date only one uveitogenic epitope for each has been identified. The purpose of this study was to identify and characterize additional uveitogenic epitopes in B10.RIII and C57BL/6 mice and to compare epitope recognition in wild-type versus IRBP-deficient mice on both backgrounds. METHODS Mice were immunized with IRBP. Spleen cells were stimulated in culture with overlapping peptides representing the entire IRBP molecule, and lymphocyte proliferative responses were measured. Peptides determined to be immunodominant were used to immunize mice for EAU. Cytokine profile and proliferation of the CD4 versus CD8 subsets were analyzed for the most pathogenic peptides. RESULTS Two new major pathogenic epitopes were identified in WT C57BL/6 mice, residues 461-480 and 651-670. These epitopes induced EAU of severity similar to that induced by the previously known peptide, 1-20. Several other peptides elicited mild disease with lower incidence. Some peptides elicited EAU only in WT recipients of IRBP KO splenocytes. In the B10.RIII strain, two major new uveitogenic peptides were identified, 171-190 and 541-560, and several others elicited moderate disease. Unlike in C57BL/6 mice, adoptive transfer of WT B10.RIII with IRBP KO splenocytes did not reveal additional uveitogenic epitopes. Both CD4 and CD8 lymphocyte subsets proliferated to pathogenic peptides. CONCLUSIONS Several new pathogenic peptides of IRBP were identified in C57BL/6 and B10.RIII mice. Differences in epitope recognition between WT and IRBP KO mice were observed in C57BL/6 mice, but not in B10.RIII mice, suggesting more extensive culling of the repertoire in C57BL/6 mice by endogenously expressed IRBP.

Functional Macrophage Heterogeneity in a Mouse Model of Autoimmune Central Nervous System Pathology

Functional macrophage heterogeneity is well appreciated outside the CNS in wound healing and cancer, and was recently also demonstrated in several CNS compartments after “sterile” insults. Yet, such heterogeneity was largely overlooked in the context of inflammatory autoimmune pathology, in which macrophages were mainly associated with disease induction and propagation. In this article, we show the diversity of monocyte-derived macrophages along the course of experimental autoimmune uveitis, an inflammatory condition affecting the ocular system, serving as a model for CNS autoimmune pathology. Disease induction resulted in the appearance of a distinct myeloid population in the retina, and in the infiltration of monocyte-derived macrophages that were absent from control eyes. During the disease course, the frequency of CX3CR1high infiltrating macrophages that express markers associated with inflammation-resolving activity was increased, along with a decrease in the frequency of inflammation-associated Ly6C+ macrophages. Inhibition of monocyte infiltration at the induction phase of experimental autoimmune uveitis prevented disease onset, whereas monocyte depletion at the resolution phase resulted in a decrease in Foxp3+ regulatory T cells and in exacerbated disease. Thus, monocyte-derived macrophages display distinct phenotypes throughout the disease course, even in an immune-induced pathology, reflecting their differential roles in disease induction and resolution.

A new look at immune privilege of the eye: dual role for the vision-related molecule, retinoic acid

The eye is an immunologically privileged and profoundly immunosuppressive environment. Early studies reported inhibition of T cell proliferation, IFN-γ production, and generation of regulatory T cells (Tregs) by aqueous humor (AH) and identified TGF-β as a critical factor. However, T cell subsets including Foxp3+ Treg and Th17 were unknown at that time, as was the role of retinoic acid (RA) in Treg induction. Consequently, the effect of the ocular microenvironment on T cell lineage commitment and function, and the role of RA in this process, had not been explored. We now use gene-manipulated mice and highly purified T cell populations to demonstrate that AH suppresses lineage commitment and acquisition of Th1 and Th17 effector function of naive T cells, manifested as reduction of lineage-specific transcription factors and cytokines. Instead, AH promoted its massive conversion to Foxp3+ Tregs that expressed CD25, GITR, CTLA-4, and CD103 and were functionally suppressive. TGF-β and RA were both needed and synergized for Treg conversion by AH, with TGF-β–enhancing T cell expression of RA receptor α. Newly converted Foxp3+ Tregs were unstable, but were stabilized upon continued exposure to AH or by the DNA demethylating agent 5-aza-2′-deoxycytidine. In contrast, T cells already committed to effector function were resistant to the suppressive and Treg-inducing effects of AH. We conclude that RA in the eye plays a dual role: in vision and in immune privilege. Nevertheless, primed effector T cells are relatively insensitive to AH, helping to explain their ability to induce uveitis despite an inhibitory ocular microenvironment.

Prior Exposure to Zika Virus Significantly Enhances Peak Dengue-2 Viremia in Rhesus Macaques

Structural and functional homologies between the Zika and Dengue viruses’ envelope proteins raise the possibility that cross-reactive antibodies induced following Zika virus infection might enhance subsequent Dengue infection. Using the rhesus macaque model we show that prior infection with Zika virus leads to a significant enhancement of Dengue-2 viremia that is accompanied by neutropenia, lympocytosis, hyperglycemia, and higher reticulocyte counts, along with the activation of pro-inflammatory monocyte subsets and release of inflammatory mediators. Zika virus infection induced detectable Dengue cross-reactive serum IgG responses that significantly amplified after Dengue-2 virus infection. Serum from Zika virus immune animals collected prior to Dengue-2 infection showed significant capacity for in vitro antibody dependent enhancement of Dengue-1, 2, 3 and 4 serotypes suggesting that pre-existing immunity to Zika virus could potentially enhance infection by heterologous Dengue serotypes. Our results provide first in vivo evidence that prior exposure to Zika virus infection can enhance Dengue infection, which has implications for understanding pathogenesis and the development of vaccines.