Immune system in a dish – One dish at a time

By Johanna Lueckel.

What if we could grow a lymph node it in the lab, and watch T and B cells go about their business? No mice, no scalpel, no drama? That’s exactly the promise of so called immune organoids. In vitro grown and matured tissues designed to mimic the structure and function of lymphoid organs. It may sound like science fiction, but researchers are already using these in vitro systems to study immune cell cross-talk, antibody production, and vaccine responses, all without the organoids ever leaving the incubator. But don’t be fooled: For scientists, this means trading animal work for very long hours in the lab. Unfortunately organoids don’t grow themselves and need constant care and affection.

In this post, we’ll take a look at what’s been achieved so far, what’s still ahead, and how these systems are reshaping experimental immunology: one dish at a time.
Let’s have a look at the bone marrow first, the birthplace of all immune cells. In vitro models of bone marrow have made real progress: 3D co-cultures of human mesenchymal stem and progenitor cells (MSPCs) and hematopoietic stem and progenitor cells (HSPCs), as well as bone-marrow-on-a-chip models support, perivascular niches, and drug testing. [1-4] More recently, bone marrow organoids have been generated from human pluripotent stem cells (PSCs) with the help of fancy hydrogels, which were consecutively engrafted into immunodeficient mice. [5]. Full replication however of the adult human bone marrow remains a work in progress. Rebuilding the thymus has also been tricky, mostly because the thymic epithelial cells (TECs), which guide T cell development, don’t thrive without the right three-dimensional and spatial structure. Still, artificial thymic organoids (ATOs) have shown promise in supporting T cell development from stem cells. [6] Novel protocols to generate mature TECs from human iPSCs entirely in vitro do not require mouse transplantation anymore. But TECs are just one puzzle piece. Vasculature, mesenchyme, and migratory cues would be required to assemble orchestrate a functional thymus-in-a-dish. What about the lymph node? Recreating its complexity in a dish is not easy but ex vivo cultures from human tonsils have laid the groundwork. [7] These models formed germinal-center-like structures and supported antigen-specific antibody production. An additional step has been the integration of lymphatic flow using microfluidic chips and bioreactors. [8] While we don’t yet have fully stem-cell-derived lymph node organoids, combining scaffolds, perfusion, and primary human immune cells has brought us impressively close. The next big leap? A self-assembling lymph node in a dish: complete with flow, follicles, and a sense of immune urgency. Unlike lymph nodes, spleen organoid models are rare and usually focus on structure or red pulp flow, rather than full immune function. Some groups have built spleen-mimicking scaffolds or used microfluidic systems to study red blood cell clearance, but models of the spleen’s white pulp and innate immune defenses are still missing. [9] There’s plenty of room to grow here: future spleen organoids could combine bone marrow-derived immune cells with engineered blood flow to mimic its pathogen-fighting abilities. Just because it’s under the radar doesn’t mean it lacks potential: the spleen definetly deserves its spotlight in the dish.

It is remarkable to witness the explosion of organoid models for lymphoid tissues. These mini-organs might open up human-specific immune processes, that we simply can’t study in mice. Still, most current models are immature and need (a lot of!) optimisation to become more functional and reproducible. Efforts to “age” organoids are still in its infancy. A critical step if we want to capture how immunity changes across the human life span. One other exciting frontier is the use of immune organoids in evaluating vaccine responses in vitro. Imagine testing immunogenicity or fine-tuning adjuvants before ever entering a clinical trial. [10] And beyond vaccines, some organoid types, like thymic organoids, may one day be scaled up for clinical applications, such as generating T cells for adoptive cell therapies like CAR T-cells. [11]

However, certain immunological questions will still need to be addressed with mouse models. Just because organoids on the rise, we can’t skip the training of an entire generation of young immunologists. The immune system is way too complex to fully recreate in vitro but years of research have greatly advanced our ability to recreate complex structures, reducing the need for animal experiments. One dish at a time.

References:

1. Leisten, I.; Kramann, R.; Ventura Ferreira, M.S.; Bovi, M.; Neuss, S.; Ziegler, P.; Wagner, W.; Knüchel, R.; Schneider, R.K. 3D Co-Culture of Hematopoietic Stem and Progenitor Cells and Mesenchymal Stem Cells in Collagen Scaffolds as a Model of the Hematopoietic Niche. Biomaterials 2012, 33, 1736–1747.
2. Dhami, S.P.S.; Kappala, S.S.; Thompson, A.; Szegezdi, E. Three-Dimensional Ex Vivo Co-Culture Models of the Leukaemic Bone Marrow Niche for Functional Drug Testing. Drug Discov. Today 2016, 21, 1464–1471.
3. Huang, X.; Zhu, B.; Wang, X.; Xiao, R.; Wang, C. Three-Dimensional Co-Culture of Mesenchymal Stromal Cells and Differentiated Osteoblasts on Human Bio-Derived Bone Scaffolds Supports Active Multi-Lineage Hematopoiesis in Vitro: Functional Implication of the Biomimetic HSC Niche. Int. J. Mol. Med. 2016, 38, 1141–1151.
4. Georgescu, A.; Oved, J.H.; Galarraga, J.H.; Cantrell, T.; Mehta, S.; Dulmovits, B.M.; Olson, T.S.; Fattahi, P.; Wang, A.; Candarlioglu, P.L.; et al. Self-Organization of the Hematopoietic Vascular Niche and Emergent Innate Immunity on a Chip. Cell Stem Cell 2024, 31, 1847–1864.e6.
5. Frenz-Wiessner, S.; Fairley, S.D.; Buser, M.; Goek, I.; Salewskij, K.; Jonsson, G.; Illig, D.; zu Putlitz, B.; Petersheim, D.; Li, Y.; et al. Generation of Complex Bone Marrow Organoids from Human Induced Pluripotent Stem Cells. Nat. Methods 2024, 21, 868–88.
6. Seet, C.S.; He, C.; Bethune, M.T.; Li, S.; Chick, B.; Gschweng, E.H.; Zhu, Y.; Kim, K.; Kohn, D.B.; Baltimore, D.; et al. Generation of Mature T Cells from Human Hematopoietic Stem and Progenitor Cells in Artificial Thymic Organoids. Nat. Methods 2017, 14, 521–530.
7. Wagar, L.E.; Salahudeen, A.; Constantz, C.M.; Wendel, B.S.; Lyons, M.M.; Mallajosyula, V.; Jatt, L.P.; Adamska, J.Z.; Blum, L.K.; Gupta, N.; et al. Modeling Human Adaptive Immune Responses with Tonsil Organoids. Nat. Med. 2021, 27, 125–135.
8. Shanti, A.; Samara, B.; Abdullah, A.; Hallfors, N.; Accoto, D.; Sapudom, J.; Alatoom, A.; Teo, J.; Danti, S.; Stefanini, C. Multi-Compartment 3D-Cultured Organ-on-a-Chip: Towards a Biomimetic Lymph Node for Drug Development. Pharmaceutics 2020, 12, 464.
9. Picot, J.; Ndour, P.A.; Lefevre, S.D.; El Nemer, W.; Tawfik, H.; Galimand, J.; Da Costa, L.; Ribeil, J.A.; de Montalembert, M.; Brousse, V.; et al. A Biomimetic Microfluidic Chip to Study the Circulation and Mechanical Retention of Red Blood Cells in the Spleen. Am. J. Hematol. 2015.
10. Wagar, L.E. Human Immune Organoids: A Tool to Study Vaccine Responses. Nat. Rev. Immunol. 2023, 23, 699.
11. Montel-Hagen, A.; Crooks, G.M. From Pluripotent Stem Cells to T Cells. Exp. Hematol. 2019, 71, 24–31.