Bacterial sugar boosts experimental cancer vaccine in mice

A sugar produced by a lactic acid bacterium has been used to build an experimental cancer vaccine that generated stronger anti-tumour immunity and improved the performance of an established immunotherapy in mice, according to reporting by Scienmag.

The platform, described by researchers in Materials Today Bio, uses an exopolysaccharide called EPS233. The substance was isolated from Lacticaseibacillus paracasei, a lactic acid bacterium, and selected from a screening programme involving 20 bacterial sugars. Researchers found that EPS233 could assemble itself into a nanoparticle while carrying both a tumour antigen and an immune-stimulating drug.

The findings address a persistent obstacle in cancer-vaccine research. Many experimental vaccines have struggled to produce enough CD8-positive T cells, which can recognise and kill malignant cells. The new approach is designed to improve that response by combining delivery and immune activation in one structure.

A self-assembling delivery system

EPS233 is amphiphilic, meaning that parts of its molecular structure interact with water while other parts avoid it. When placed in water, the sugar formed spherical particles approximately 27 nanometres wide. The researchers said this size could help the particles travel to lymph nodes, where immune responses are organised.

The exopolysaccharide had an average molecular mass of about 77.6 kilodaltons and contained a backbone rich in mannose. That composition was important because the sugar could stimulate immune signalling as well as transport the vaccine’s other components.

For the vaccine formulation, the team paired EPS233 with ovalbumin, used as a model tumour antigen, and resiquimod, or R848. R848 activates the innate immune receptors TLR7 and TLR8, but free versions of the compound can cause excessive inflammation. The combined formulation, called EPS@R848/OVA, produced particles slightly larger than 37 nanometres and enclosed about 84% of each payload.

Laboratory modelling indicated that the sugar and R848 could bind naturally, helping the three ingredients remain together. Release tests suggested that the particles stayed relatively stable at normal physiological pH but opened more readily in the acidic environment of cellular lysosomes. More than half of the antigen was released within 24 hours under those conditions, allowing it to become available inside dendritic cells, the immune cells responsible for initiating T-cell responses.

Stronger activation of immune cells

The delivery system also appeared to reduce R848’s harmful systemic effects. Mice receiving free R848 with antigen experienced a rise in inflammatory cytokines within six hours and temporary weight loss. Those effects were not observed with the nanoparticle formulation, which seemed to retain the drug and release it more gradually.

Tests on dendritic cells and fibroblasts found little toxicity at therapeutic concentrations. The particles also caused no more damage to red blood cells than saline in haemolysis tests. Repeated treatment produced no visible abnormalities in the heart, liver, spleen, lungs or kidneys, while blood chemistry remained normal.

Once inside dendritic cells, the vaccine was rapidly taken up and gradually moved out of lysosomes. This mattered because antigens confined within lysosomes may be broken down rather than displayed to T cells. The treated cells showed increased levels of antigen-bearing MHC-I molecules and the activation markers CD40, CD80 and CD86. They also released several immune signalling proteins, including interferon-beta, IL-6, TNF-alpha and IL-12p70.

The researchers linked these effects to two cooperating pathways. EPS233 activated the Dectin-2 receptor and its Syk-CARD9 signalling route, while R848 stimulated the TLR7-MyD88-IRF7 pathway. Together, they increased type I interferon activity and improved the process by which dendritic cells present tumour antigens to CD8-positive T cells. Gene-expression studies also found higher levels of Irf8 and Batf3, which help produce the dendritic-cell population specialised in cross-presentation.

In mice, the particles remained near the injection site for more than 120 hours but reached nearby lymph nodes within six hours. They were taken up by dendritic cells, macrophages and B cells, including a rare dendritic-cell subset considered particularly effective at activating killer T cells. A mixture of the same ingredients without nanoparticle assembly did not produce equivalent results, suggesting that the structure itself was crucial.

Tumour control and checkpoint therapy

Vaccinated mice developed larger populations of activated and multifunctional CD8-positive T cells, as well as Th1-type CD4 responses, germinal-centre B cells and antigen-specific antibodies. In preventive melanoma experiments, animals challenged with B16F10-OVA cells had approximately 52% lower tumour weight than mice given the separate components.

The treatment also worked against established tumours. In a therapeutic melanoma model, it reduced average tumour volume by about half compared with single-adjuvant controls. Tumours contained more activated CD8 and CD4 cells producing interferon-gamma, TNF-alpha and granzyme B, alongside fewer regulatory T cells and myeloid-derived suppressor cells. A version using the GP33 peptide also reduced lung metastatic nodules.

The most notable results came when the vaccine was combined with anti-PD-1 therapy. In an orthotopic B16F10-GP33 melanoma model, the combination reduced tumour weight by 98.1% and extended median survival to 41 days, compared with 32 days for anti-PD-1 alone. In a lung-metastasis model, tumour burden fell by 88.1%, while median survival reached 33 days versus 25 days with antibody treatment alone.

The researchers suggest that the vaccine may make poorly inflamed tumours more accessible to checkpoint therapy by supplying an active T-cell response. The study remains an early proof of concept: it used model antigens and melanoma in mice, rather than personalised neoantigens or established human tumour targets. Those tests will be needed to determine whether a probiotic-derived sugar can eventually support a cancer vaccine for clinical use.