The human body has around 600 lymph nodes (LNs) scattered throughout it, small, bean-shaped organs that house various types of blood cells and filter lymph fluid which temporarily swell during infections with viruses or other pathogens. This LN expansion and subsequent contraction can also result from vaccines injected nearby, and in fact is thought to reflect the ongoing vaccine immune response. While researchers have studied the early expansion of LNs following vaccination, they have not investigated whether prolonged LN expansion could affect vaccine outcomes.
Now, for the first time, researchers from Harvard University and the company Genentech found a way to enhance and extend LN expansion, and study how this phenomenon affects both the immune system and efficacy of vaccinations against tumours.
Key to their approach was a biomaterial vaccine formulation that enabled greater and more persistent LN expansion than standard control vaccines. While the oversized LNs maintained a normal tissue organization, they displayed altered mechanical features and hosted higher numbers of various immune cell types that commonly are involved in immune responses against pathogens and cancers. Importantly, “jump-starting” lymph node expansion prior to administering a traditional vaccine against a melanoma-specific model antigen led to more effective and sustained anti-tumour responses in mice. The findings are published in Nature Biomedical Engineering.
“By enhancing the initial and sustained expansion of LNs with biomaterial scaffolds, non-invasively monitoring them individually over long time periods, and probing deeply into their tissue architecture and immune cell populations, we tightly correlate a persistent LN expansion with more robust immune and vaccination responses,” said Wyss Institute Founding Core Faculty member David Mooney, Ph.D., who led the study. “This opens a new front of investigation for immunologists, and could have far-reaching implications for future vaccine developments.” Mooney also is the Robert P. Pinkas Family Professor of Bioengineering at SEAS, and a co-principal investigator of the NIH-funded and Wyss-coordinated Immuno-Engineering to Improve Immunotherapy (i3) Center.
The research team had previously developed biomaterial vaccine formulations, but had not investigated how their vaccines and those developed by others could influence the response of LNs draining leaked tissue fluid at vaccine injection sites, and have an impact on the LNs tissue organisation, different cell types, and their gene expression, which could in turn affect vaccine efficacy. In their new study, they tested a previously developed vaccine formulation that is based on microscale mesoporous silica (MPS) rods that can be injected close to tumours and form a cell-permeable 3D scaffold structure under the skin. Engineered to release an immune cell-attracting cytokine (GM-CSF), and immune cell-activating adjuvant (CpG), and tumour-antigen molecules, MPS-vaccines are able to reprogram recruited so-called antigen-presenting cells that, upon migrating into nearby LNs, orchestrate complex tumour cell-killing immune responses. Their new study showed that there are more facets to that concept.
“As it turns out, the immune-boosting functions of basic MPS-vaccines actively change the state of LNs by persistently enlarging their whole organ structure, as well as changing their tissue mechanics and immune cell populations and functions,” said first-author Alexander Najibi, PhD, who performed his Ph.D. thesis with Mooney.
Probing LNs with ultra-sound and nano-devices
Using high-frequency ultrasound, the team traced individual LNs in MPS-vaccinated mice over 100 days. They identified an initial peak expansion period that lasted until day 20, in which LN volumes increased about 7-fold, significantly greater than in animals that received traditional vaccine formulations. Importantly, the LNs of MPS-vaccinated mice, while decreasing in volumes after this peak expansion, remained significantly more expanded than LNs from traditionally vaccinated mice throughout the 100-day time course.
When Najibi and the team investigated the mechanical responses of the LNs using a nanoindentation device, they found that LNs in MPS-vaccinated animals, although maintaining an overall normal structure, were less stiff and more viscous in certain locations. This was accompanied by a re-organisation of a protein that assembles and controls cells’ mechanically active cytoskeleton. Interestingly, Mooney’s group had shown in an earlier biomaterial study that changing mechanical features of immune cells’ environments, especially their viscoelasticity, affects immune cell development and functions. “It is very well-possible that in order to accommodate the significant growth induced by MPS-vaccines, LNs need to become softer and more viscous, and that this then further impacts immune cell recruitment, proliferation, and differentiation in a feed-forward process,” said Najibi.
From immune cell engagement to vaccine responses
Interestingly, upon MPS-vaccination, the numbers of “innate immune cells,” including monocytes, neutrophils, macrophages, and other cell types that build up the first wave of immune defences against pathogens and unwanted cells, peaked first in expanding LNs. Peaking with a delay were dendritic cells (DCs), which normally transfer information in the form of antigens from invading pathogens and cancer cells to “adaptive immune cells” that then launch subsequent waves of highly specific immune responses against the antigen-producing invaders. In fact, along with DCs, also T and B cell types of the adaptive immune system started to reach their highest numbers. “It was fascinating to see how the distinct changes in immune cell populations that we detected in expanding LNs in response to the MPS-vaccine over time re-enacted a typical immune response to infectious pathogens,” commented Najibi.
Innate immune cells and DCs are also known as “myeloid cells,” which are known to interact with LN tissue during early expansion. To further define the impact of myeloid cells on LN expansion, Mooney’s team collaborated with the group of Shannon Turley, PhD, the VP of Immunology and Regenerative Medicine at Genentech, and an expert in lymph node biology and tumour immunology. “The MPS-vaccine led to extraordinary structural and cellular changes within the lymph node that supported potent antigen-specific immunity,” said Turley.
Using single cell RNA sequencing on myeloid cells from LNs, the groups were able to reconstruct distinct changes in myeloid cell populations during LN expansion, and identified distinct DC populations in durably expanded LNs whose changed gene expression was associated with LN expansion. In addition, the collaborators found that the number of monocytes was increased 80-fold upon MPS-vaccination – the highest increase among all myeloid cell types – and pinpointed subpopulations of “inflammatory and antigen-presenting monocytes” as promising candidates for facilitating LN expansion. In fact, when they depleted specific subpopulations of these types of monocytes from circulating blood of mice after vaccination, the maintenance of LN expansion, and timing of the T cell response to vaccination, was altered.
Finally, the team explored whether LN expansion could enhance the effectiveness of vaccination. “Jump-starting” the immune system in LNs with an antigen-free MPS-vaccine and subsequently administering the antigen in a traditional vaccine format significantly improved anti-tumour immunity and prolonged the survival of melanoma-bearing mice, compared to the traditional vaccine alone. “The priming of lymph nodes for subsequent vaccinations using various formulations could be a low-hanging fruit for future vaccine developments,” said Mooney.
Source: Wyss Institute for Biologically Inspired Engineering at Harvard
