Immunodeficient mice engrafted with functional human cells and tissues, that is, humanized mice, have become increasingly important as small, preclinical animal models for the study of human diseases. Since the description of immunodeficient mice bearing mutations in the IL2 receptor common gamma chain (IL2rgnull) in the early 2000s, investigators have been able to engraft murine recipients with human hematopoietic stem cells that develop into functional human immune systems. These mice can also be engrafted with human tissues such as islets, liver, skin, and most solid and hematologic cancers. Humanized mice are permitting significant progress in studies of human infectious disease, cancer, regenerative medicine, graft-versus-host disease, allergies, and immunity. Ultimately, use of humanized mice may lead to the implementation of truly personalized medicine in the clinic. This review discusses recent progress in the development and use of humanized mice and highlights their utility for the study of human diseases.

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Humanized Mouse Models of Clinical Disease.

It has long been recognized that the mammalian gut microbiota has a role in the development and activation of the host immune system. Much less is known on how host immunity regulates the gut microbiota. Here we investigated the role of adaptive immunity on the mouse distal gut microbial composition by sequencing 16 S rRNA genes from microbiota of immunodeficient Rag1−/− mice, versus wild-type mice, under the same housing environment. To detect possible interactions among immunological status, age and variability from anatomical sites, we analyzed samples from the cecum, colon, colonic mucus and feces before and after weaning. High-throughput sequencing showed that Firmicutes, Bacteroidetes and Verrucomicrobia dominated mouse gut bacterial communities. Rag1− mice had a distinct microbiota that was phylogenetically different from wild-type mice. In particular, the bacterium Akkermansia muciniphila was highly enriched in Rag1−/− mice compared with the wild type. This enrichment was suppressed when Rag1−/− mice received bone marrows from wild-type mice. The microbial community diversity increased with age, albeit the magnitude depended on Rag1 status. In addition, Rag1−/− mice had a higher gain in microbiota richness and evenness with increase in age compared with wild-type mice, possibly due to the lack of pressure from the adaptive immune system. Our results suggest that adaptive immunity has a pervasive role in regulating gut microbiota’s composition and diversity.

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Host adaptive immunity alters gut microbiota

Human immunodeficiency virus (HIV) disease is associated with dysregulation and dysfunction involving all major lymphocyte populations, including B cells. Such perturbations occur early in the course of infection and are driven in large part by immune activation resulting from ongoing HIV replication leading to bystander effects on B cells. While most of the knowledge regarding immune cell abnormalities in HIV-infected individuals has been gained from studies conducted on the peripheral blood, it is clear that the virus is most active and most damaging in lymphoid tissues. Here, we discuss B-cell perturbations in HIV-infected individuals, focusing on the skewing of B-cell subsets that circulate in the peripheral blood and their counterparts that reside in lymphoid tissues. This review also highlights recent advances in evaluating HIV-specific B-cell responses both in the memory B-cell compartment, as well as in circulating antibody-secreting plasmablasts and the more differentiated plasma cells residing in tissues. Finally, we consider how knowledge gained by investigating B cells in HIV-infected individuals may help inform the development of an effective antibody-based HIV vaccine.

Insights into B cells and HIV-specific B-cell responses in HIV-infected individuals.

Immunoglobulin A (IgA) is the most abundant antibody class present at mucosal surfaces. The production of IgA exceeds the production of all other antibodies combined, supporting its prominent role in host-pathogen defense. IgA closely interacts with the intestinal microbiota to enhance its diversity, and IgA has a passive protective role via immune exclusion. Additionally, inhibitory ITAMi signaling via the IgA Fc receptor (FcαRI; CD89) by monomeric IgA may play a role in maintaining homeostatic conditions. By contrast, IgA immune complexes (e.g., opsonized pathogens) potently activate immune cells via cross-linking FcαRI, thereby inducing pro-inflammatory responses resulting in elimination of pathogens. The importance of IgA in removal of pathogens is emphasized by the fact that several pathogens developed mechanisms to break down IgA or evade FcαRI-mediated activation of immune cells. Augmented or aberrant presence of IgA immune complexes can result in excessive neutrophil activation, potentially leading to severe tissue damage in multiple inflammatory, or autoimmune diseases. Influencing IgA or FcαRI-mediated functions therefore provides several therapeutic possibilities. On the one hand (passive) IgA vaccination strategies can be developed for protection against infections. Furthermore, IgA monoclonal antibodies that are directed against tumor antigens may be effective as cancer treatment. On the other hand, induction of ITAMi signaling via FcαRI may reduce allergy or inflammation, whereas blocking FcαRI with monoclonal antibodies, or peptides may resolve IgA-induced tissue damage. In this review both (patho)physiological roles as well as therapeutic possibilities of the IgA-FcαRI axis are addressed.

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IgA and FcαRI: Pathological Roles and Therapeutic Opportunities.

Immunodeficient mice engrafted with human cells and tissues have provided an exciting alternative to in vitro studies with human tissues and nonhuman primates for the study of human immunobiology. A major breakthrough in the early 2000s was the introduction of a targeted mutation in the interleukin 2 (IL-2) receptor common gamma chain (IL2rgnull) into mice that were already deficient in T and B cells. Among other immune defects, natural killer (NK) cells are disrupted in these mice, permitting efficient engraftment with human hematopoietic cells that generate a functional human immune system. These humanized mouse models are becoming increasingly important for preclinical studies of human immunity, hematopoiesis, tissue regeneration, cancer, and infectious diseases. In particular, humanized mice have enabled studies of the pathogenesis of human-specific pathogens, including human immunodeficiency virus type 1, Epstein Barr virus, and Salmonella typhi. However, there are a number of limitations in the currently available humanized mouse models. Investigators are continuing to identify molecular mechanisms underlying the remaining defects in the engrafted human immune system and are generating “next generation” models to overcome these final deficiencies. This article provides an overview of some of the emerging models of humanized mice, their use in the study of infectious diseases, and some of the remaining limitations that are currently being addressed.

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Overcoming current limitations in humanized mouse research.

Inhibitory effect of HIV-specific neutralizing IgA on mucosal transmission of HIV in humanized mice.

Problem:
To determine the interplay between fetal antigenicity and local maternal factors in determining reproductive tract T regulatory (Treg) cell accumulation during pregnancy.
Method of study:
Examination of maternal Treg composition in the uterus, cervix, and uteroplacental interface (UPI) of murine syngeneic and allogeneic pregnancies and non-pregnant controls by flow cytometry. The impact of fetal antigenicity was defined by either fetal gender in syngeneic pregnancies or by allogeneic paternity. Impact of IL-6 on local Treg composition was determined using syngeneic pregnancies in IL-6^(−/−) females.
Results:
An increased fraction of CD4^+ T cells in the pregnant uterine lymphocytic infiltrate and draining pelvic lymph nodes are Tregs. Maternal IL-6 decreases Treg accumulation within the uterus and to a greater extent in the cervix in syngeneic pregnancy. Fetal antigenicity is matched by accumulation of Tregs to the UPI. Treg accumulation at the UPI of non-antigenic female fetuses is determined by the intrauterine position relative to male siblings.
Conclusion:
Reproductive tract tissue Treg composition during pregnancy is influenced by maternal IL-6 and fetal antigenicity.

Maternal and fetal factors that contribute to the localization of T regulatory cells during pregnancy.

Background: HIV/AIDS remains one of the most serious current
threats to global public health. Although anti-HIV drugs
have been effective among the wealthiest populations, a vaccine and/or new methods to prevent infections are needed lo control HIV globally. Strategies to combat HIV-1 require structural knowledge of how antibodies recognize HIV envelope proteins and how the immune system eliminates viruses. Until recently, only a small number of broadly neutralizing antibodies against HIV-1 had been characterized, and the immunological basis for their breadth and potency remains poorly understood. However.
it was recently demonstrated that antibodies could be engineered to greatly enhance their breadth and potency (Diskin et al., Science 2011). Unfortunately, this and other engineering efforts have resulted in a decrease in antibody half-life in mouse and non-human primate models. This decrease in half-life correlates with an increase in reactivity to a variety of antigens, termed polyreactivity.
Methods: In order to make better targets for passive delivery therapies, we are working to increase the half-life of antibodies while maintaining their breadth and potency using a variety of computational and structured-based techniques. One technique involves
reducing the spatial aggregation propensity, in which an
algorithm finds dynamically exposed hydrophobic patches on the surlace of proteins (Chemansetty et al., PNAS 2009). To this end, we have constructed several mutations in regions that have been predicted to have high aggregation propensities, and have tested them for polyreactivity and potency in neutralization assays.
Results: Initial results show that these novel reagents have reduced polyreactivily, yet they still maintain their potency in in vitro neutralization assays.
Conclusions: We are currently pursuing in vivo experiments in mice to further understand the relationship between antibody potency, polyreactivity, and half-life.

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Sonal N. Patel

Papers

Inhibitory effect of HIV-specific neutralizing IgA on mucosal transmission of HIV in humanized mice.

Maternal and fetal factors that contribute to the localization of T regulatory cells during pregnancy.

Engineering Antibodies to Enhance Activity and Increase Half-life