Characterization of OVA-ND complexes. NDs before and after mixing with OVA were analyzed for their size distributions and zeta potentials. Transmission electron microscopy (TEM) of bare NDs first revealed that the particles were irregular in shape and varied considerably in size (inset in Fig. 1). Dynamic light scattering measurements showed that bare NDs have a mean hydrodynamic diameter of ~ 100 nm and a zeta potential of − 45 mV (Fig. 1). The average diameter of the particles increased by about 20 nm after mixing with OVA in water, indicating that these NDs have been successfully coated with OVA by physical adsorption. As the isoelectric point of OVA is 4.5 [29], meaning that the protein molecules are negatively charged in PBS buffer (pH 7.4), the change of the zeta potential from − 45 mV of ND to − 23 mV of OVA-ND implied that forces (such as hydrophobic forces) other than electrostatic interaction play important roles in the protein adsorption process.
As a member of nanocarbon family, the surface of NDs can be conveniently modified with a variety of oxygen-containing groups such as –COOH, –COH, –COOC–, etc. by extensive washes in strong oxidative acids. Uniquely, the acid-washed NDs exhibit an exceptionally high affinity for a wide range of protein molecules including bovine serum albumin (BSA), myoglobin, cytochrome c, lysozyme, and luciferase [30–32]. Moreover, the structural integrity of these proteins is retained, as demonstrated by the catalytic activities of lysozyme and luciferase after adsorption on NDs [31, 32]. Chicken OVA is a phosphorylated glycoprotein consisting of 385 amino acid residues with a molecular weight of 42.7 kDa or a total molecular weight of 45 kDa including the carbohydrate and phosphate portions [29]. To evaluate the amount of OVA that could be loaded on the acid-washed NDs, we measured the changes in optical absorbance of unbound OVA at 280 nm before and after mixing with the nanoparticles (Supplementary Figure S1). For OVA adsorbed on 100-nm NDs, we determined a protein loading capacity of OVA:ND = 1:8 (weight ratio) at saturation. Assuming a spherical shape for the adsorbent, this high loading capacity suggests that each 100-nm ND (weight of ~ 1.8 fg/particle) can accommodate more than 3000 OVA molecules on surface.
Immune responses. The in vivo experiments were started by mixing 5 µg OVA with 60 µg NDs and dispersing the OVA-conjugated NDs in PBS, CFA, or IFA prior to subcutaneous injection of the mixtures into BALB/C mice. The corresponding control experiments consisted of 5 µg OVA in PBS, CFA, or IFA, respectively (Figs. 2 and 3). Figure 2A shows the timeline of immunization and blood collection in this experiment. The water-in-oil emulsions formed small nodules and appeared as soft capsules at the injection sites upon immunization. We evaluated OVA-specific IgG antibody responses in the sera of the immunized mice with enzyme-linked immunosorbent assays (ELISA) after the second and third immunizations with OVA and OVA/ND in CFA. As shown in Fig. 2B and 2C, the OVA/ND/CFA treatments induced a significantly higher amount of OVA-specific IgG antibodies in the mouse sera than OVA/CFA alone by 3.5- and 1.6-fold, respectively, after the second and third immunizations. It is demonstrated that the addition of NDs in CFA is able to elicit highly efficient and protective immune responses against OVA in the mouse body, in line with a previous report that NDs can enhance immune responses against recombinant HA/H7N9 in mice [33].
Next, we investigated the dose dependence of the immune response by employing OVA/CFA and OVA/ND/CFA containing 25 µg OVA. The amount of NDs used accordingly increased to 300 µg. Indeed, a 2-fold increase of the OVA-specific IgG antibody production was found in the OVA/CFA treatment (Fig. 2A). However, the response did not exceed that of the 5-µg treatment with OVA/ND/CFA. Notably, further increase of the OVA dose failed to boost the immune response in the OVA/ND/CFA treatment. The result suggested a saturation effect, where no higher levels of anti-OVA could be reached irrespective of the doses of OVA applied. An important implication of this finding is that the use of NDs as additives in CFA can help reduce the consumption of antigens in producing the antibodies of interest, which is a valuable feature for industrial production of antibodies.
We explored further if the same level of immune response by OVA/ND/CFA could be maintained without the need of allergic components such as inactivated mycobacteria in CFA. The dosage groups of 5 µg and 25 µg OVA were tested in parallel in this experiment. As shown in Fig. 3, we did not found significant differences in the results between OVA/ND/IFA and OVA/ND/CFA treatments in these two groups, indicating that the substitution of dead mycobacteria by NDs as additives in the mineral oil not only can improve the safety but also can maintain the efficacy of the vaccine adjuvant. This new combination of substances is expected to work well also as immune drug delivery vehicles to promote directed antitumor activities with minimal systemic toxicity [27].
Antitumor therapeutics. The new formulation of NDs in oil emulsions is applicable as antitumor therapeutic agents as well. To demonstrate the application, we employed the mouse lymphoma cell lines, EL4 and E.G7-OVA. The E.G7-OVA cells are able to express OVA and have been widely used in cancer immunotherapy studies. Depicted in Fig. 4A is the timeline for the injection of OVA/ND/IFA, followed by the inoculation of EL4 and E.G7-OVA cells in C57BL/6 mice. By referring to the unvaccinated groups, we found that the treatment with OVA/ND/IFA in the EL4 model was unable to delay the tumor growth (Fig. 4B). In contrast, the OVA/ND/IFA treatment could effectively inhibit the tumor progression in the E.G7 model over 3 weeks post inoculation of the cells (Fig. 4C). Notably, half of the mice (4 out of 7 mice) in the E.G7 model maintained tumor-free for more than 15 days after cell inoculation (Fig. 4D) and survived up to 35 days post tumor cell challenges (Fig. 4E). In Fig. 4F, we show photographs of the tumors isolated on day 24 from vaccinated and non-vaccinated mice. The difference in tumor size between these two groups (in triplicate) of mice is substantially, about 10 times in total volume. Taken together, these results indicate that the presently developed nanovaccines with ND/IFA as adjuvants are promising agents for cancer immunotherapy.
To further assess the therapeutic potential of OVA/ND/IFA, we investigated the in vivo immunostimulatory activity of the agent with just one dose in each mouse. Single-dose therapy has several advantages over multiple-dose therapy, including greater patient compliance, less risk of side effects, and lower costs [34]. In particular, knowing the effectiveness of the single-dose vaccines composed of either whole viruses, protein subunits, viral vectors, or nucleic acids (RNA and DNA) is critically important in the prevention and control of COVID-19 infections today [35]. Additionally, in protecting livestock (such as cattle, sheep, pigs, and goats) from infectious diseases, single-dose veterinary vaccine makes it easier for suppliers to streamline the production process and distribution of the agents to rural areas [36].
In this single-shot experiment, mice were first administrated with OVA/ND/IFA via subcutaneous injection and then examined by measuring the production of anti-OVA IgG in the mouse sera on a weekly basis. We found that the OVA/ND/IFA treatment could dramatically induce the production of OVA-specific IgG antibodies on day 28 and day 35 after the administration (Fig. 5). Compared with the OVA/ND and OVA/IFA groups using the same amount of antigens, the OVA/ND/IFA treatment boosted the levels of anti-OVA IgG by 432 and 6 times on day 28, respectively. The enhancement factor further increased to 1717 and 19 times on day 35. It is demonstrated that the addition of NDs can greatly improve the effectiveness of IFA as a single-dose vaccine adjuvant, which is capable of sustaining its immunostimulatory activities over an extended period of time.
Finally, we explored whether or not the addition of NDs in IFA altered the mechanism of the immune response elicited by IFA alone, which is known to proceed predominantly through the Th2 pathway (i.e. humoral immune response) [17, 37]. We addressed the question by performing ELISA assays for cytokines in the sera of C57BL/6 mice after injection with OVA/ND/IFA. As shown in Fig. 6, only a small difference in the interleukin 2 (IL-2) level was found between the control and treatment groups, whereas a marked elevation of the interleukin 4 (IL-4) concentration in the vaccinated group was detected. Furthermore, by replacing NDs with FNDs in the adjuvants, we were able to clearly identify the presence of FNDs in mouse spleens through background-free detection of far-red fluorescence at ~ 700 nm in the tissue digests (Supplementary Figure S2 and ref. [38] for details). All the results led us to a possible predominant mechanism for the initiation of the immune response by the ND/IFA-based vaccine as follows: (i) formation of nodules with loose structure in mouse tissues after subcutaneous injection of the antigen-loaded ND/IFA emulsion, in which the adjuvants act as a depot; (ii) sustained release of the antigens from NDs in the water phase of the emulsions; (iii) active and continuous recruitment of immature immune cells to the depot; (iv) uptake of the antigen-loaded NDs by the immune cells through endocytosis; and (v) promotion of Th2 response, where helper T cells bind with the antigen presenting cells and activate the development of B cells into antibody-producing plasma cells in spleens. The proposed mechanism is depicted in Fig. 7.