Immunity-on-a-chip
The immune system is very complex, and as the COVID-19 pandemic has shown, science still does not fully understand the complex mechanisms that protect the body from microorganisms. Why do some carry SARS-CoV-2 infection asymptomatically, while others suffer from severe fever? Why are not all patients susceptible to cytokine storm? Immunologists still do not have exact answers to these questions.
The work of a group of researchers from the Wyss Institute at Harvard University is likely to help unravel the secrets of the immune system. They cultured human B and T lymphocytes inside a microfluidic chip, forcing them to spontaneously form functional lymphatic follicles similar to those found in human lymph nodes and mediate immune responses. The follicle represents a cavity in which "inexperienced" B and T lymphocytes are located, together they initiate a cascade of events leading to an immune response when exposed to an antigen.
With the help of the new model, it will be possible not only to study the normal function of the immune system, but also to predict reactions to various vaccines in order to choose the best ones. Researchers suggest that "immunity-on-a-chip" will provide more reliable information than animal models. The immune system of animals differs significantly from that of humans, so the results obtained in animal models may not be repeated in clinical studies.
Accidental discovery
The aim of the researchers was to study how B- and T-lymphocytes circulating in the blood change their behavior when they enter the tissue. They obtained these cells from human blood samples and cultured them inside a microfluidic chip to reproduce the physical conditions of a particular organ.
When the cells were placed in one of the two channels inside the device and a culture medium was introduced for nutrition, the researchers noted that B- and T-lymphocytes began to spontaneously organize into three-dimensional structures inside the chip. Cavities resembling lymphatic follicles with a similar germinal center appeared. The unexpected behavior of lymphocytes completely changed the original purpose of the experiment, and the researchers focused on a deeper study of the lymphatic follicle on a chip (LF Chip).
Lymphocytes cultured in the fluid flow spontaneously organize into lymphoid follicles (left), the same cells do not form any structures under static conditions (right). Source: The Wyss Institute at Harvard University.
The researchers found that lymphocytes in the LF Chip secrete chemokine CXCL13, which is a characteristic feature of the formation of lymphatic follicles both in lymph nodes and in other parts of the body in response to chronic inflammation, for example, in cancer and autoimmune diseases.
The team also found that B cells inside the LF Chip express activation-induced cytidine deaminase (AID), an enzyme that is critical for activating B cells against specific antigens and is not present in B cells circulating in the blood. Neither CXCL13 nor AID were present in cells cultured in a two-dimensional medium, which means that scientists have indeed successfully created functional lymphatic follicles from circulating blood cells.
In lymphatic follicles in the human body, activated B lymphocytes mature and differentiate into several types of descendant cells, including plasma cells that secrete a large number of antibodies against a specific pathogen. The group detected the presence of plasma cells in the LF Chip after stimulating them with a combination of IL-4 cytokine and anti-CD40 or killed microorganisms. It is noteworthy that plasma cells were concentrated in clusters inside the follicles, as it would be in vivo.
Predicting the effectiveness of vaccines
The next step was to study the reliability of the data obtained using the LF Chip to study the response of the human immune system to vaccines.
In the human body, after vaccination, dendritic cells begin to absorb the introduced pathogens and migrate to the lymph nodes, where they represent their fragments on their surface. There, in the follicles, these antigen-presenting cells activate B cells with the help of T cells, causing B cells to differentiate into plasma cells that produce antibodies against the pathogen. To replicate this process, the researchers added dendritic cells to the LF Chip along with B and T cells from four separate donors. They then inoculated the chips with the H5N1 flu vaccine along with the SWE adjuvant, which is known to enhance the immune response to the vaccine.
The chips that received the vaccine and adjuvant produced significantly more plasma cells and influenza antibodies than B and T cells grown in two-dimensional cultures, or chips that received the vaccine without an adjuvant.
The team then repeated the experiment with cells from eight donors, this time using the commercially available Fluzone flu vaccine, which protects against three different strains of the virus. And again, plasma cells and anti-influenza antibodies were present in these chips in significant quantities. The researchers also measured the levels of four cytokines in the vaccinated chips, which are known to be secreted by activated immune cells, and found that the levels of three of them (IFN-γ, IL-10 and IL-2) were similar to the levels found in the serum of people vaccinated with Fluzone.
In the context of the COVID-19 pandemic, the suddenly increased demand for vaccines has made traditional animal models a scarce resource. The new chip provides cheaper, faster and more accurate modeling to study human immune responses to both infections and vaccines. Researchers are currently using the LF Chip to test various vaccines and adjuvants in collaboration with pharmaceutical companies and the Gates Foundation.
Article by G.Goyal et al. Ectopic Lymphoid Follicle Formation and Human Seasonal Influenza Vaccination Responses Recapitulated in an Organ-on-a-Chip is published in the journal Advanced Science.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on the materials of the Wyss Institute: The immune system is very complicated, but now, it’s on a chip.