Vaccination is like "drinking milk tea"! Take stock of the advantages and difficulties of inhalation seedlings

In the long history of mankind's struggle against infectious diseases, epidemics are a favorable weapon for mankind to fight infectious diseases, and accurately delivering effective vaccines into the human body is the first step to
complete the immunization process.
From the advent of subcutaneous injection in the mid-to-late 19th century to the emergence of more and more advanced syringes later, injection has become the main route of
immunization now.

However, the requirements for injecting seedlings are higher: seedlings are often formulated as unstable liquids that require refrigeration, or lyophilized powders for resolubilization; Injections require professionally trained technicians; Needlestick injuries and needle reuse increase the likelihood of cross-contamination; Finally, due to needle phobia and pain associated with injections, especially pediatrics, compliance with vaccination vaccinations is reduced
.
In addition, injecting the vaccine mainly induces a systemic immune response, rather than targeting the infected area
of the pathogen.

Most pathogens enter the body through mucosal surfaces, and nearly 80% of total immune cells in healthy adults are associated with
mucosal surfaces.
Different from injection immunity, mucosal immunity can not only induce systemic response, but also stimulate the response of mucosal tissue-resident T cells well, and promote the secretion of immunoglobulin A (IgA) antibodies by the mucosa, which is in the establishment of mucosal immune memory, thereby more effectively preventing mucosal pathogen infection
.
Mucosal disease also has the advantages of needle-free administration, such as lower risk of infection, higher compliance of vaccinated people, and lower technical requirements
.

At present, although there are intestinal and nasal mucosal immune vaccines, there are only a few of them (4 intestinal mucosa, 1 nasal mucosal influenza vaccine, see Figure 1).

Mucosal infections also have their limitations, and most of these seedlings are attenuated or inactivated seedlings, and there are hidden dangers
such as incomplete inactivation and virulence recovery.

In recent years, inhaled seedlings have become a hot spot
in research and development in the prevention of respiratory microbial infections.
The respiratory mucosa has a surface area of about 70-100 square meters, which has the characteristics of good epithelial permeability and high perfusion, and is one of
the best targets for biopharmaceutical absorption.
In particular, the emergency use authorization of an inhaled coronavirus vaccine in China has once again focused our attention on this direction
.
The study of inhaled seedlings is first of all to establish a further deepening
of the understanding of respiratory mucosal immunity.

The long-term continuous exposure of the respiratory mucosa to foreign bodies establishes a set of balanced mechanisms
for whether to induce immune response to protect the body or induce immune tolerance in the face of foreign bodies.
Normally, the respiratory tract is immune to inhalation of foreign bodies such as dust particles and other inert substances, which are cleared by mucociliary or by the cough/sneeze reflex
.

However, when pathogens invade, the respiratory mucosa has a "triple" immune protection mechanism
.
The first is the conventional pathway: APC cells (antigen-presenting cells) recognize and absorb antigens
by detecting pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMP).
Thereafter, DC cells (dendritic cells) typically migrate through afferent lymphatic vessels to nearby draining lymph nodes, where antigens are presented to naïve T cells and B cells
via the MHC-II complex.

In addition to this routine antigen presentation pathway, there is a "second" protection against specialized tissues
called inducible bronchi-associated lymphoid tissue (BALT) in human respiratory infections.
The basic structure of BALT consists of lymphatic follicles in the center, the mucosal cells that cover them, and some mutated epithelial cells, and BALT can present antigens to naïve T cells and effector T and B cells without migrating
through the lymphatic system.
There is a similar tissue near the nasal mucosa—nasal associated lymphoid tissue (NALT).

BALT and NALT are jointly involved in the mucosal immune system of the respiratory tract, triggering a humoral immune response, which is very important
for the effective treatment of pathogenic antigens and subsequent neutralization of pathogens.
The humoral immune response triggered by BALT is mainly the response of local B cells and the response of IgA, and IgA antibodies in mucosal tissues are usually dimers, and the final secretory IgA (SIgA) can cross-connect pathogens in the inner lumen, resulting in spatial barriers, thereby blocking infectivity, or it can bind to antigens in infected cells and excrete
them through vesicle transport.

The "third" protective mechanism of mucosal tissue is that antigen initiation in one specific mucosal region induces responses
in different mucosal regions.
In short, antigen activation in the nasal or oral mucosa may induce mucosal antibodies in the mucosal tissue of the lungs and vice versa
.
In other words, if a person is infected through the nasal mucosa, this can also lead to elevated
SIgA levels in the airway mucosa.
This interaction of different mucosal sites is often referred to as the common mucosal immune system
.

Structure determines nature, and the special immune system of the respiratory mucosa determines its role in
immunization.
A successful inhaled vaccine needs to meet the following requirements: the vaccine can be accurately delivered to the lungs; sufficient immunogenic activity to ensure adequate (mucosa) immune cell activation, i.
e.
it should overcome immune tolerance disorders; The epidemic has few adverse reactions, especially does not cause asthma and COPD, and the following obstacles
must be overcome in the development of vaccines.

1 Moderate immune tolerance barrier

Since the respiratory mucosa needs to maintain a certain degree of immune tolerance, ensure that hypersensitivity does
not occur during continuous exposure to a large number of harmless substances from the external environment.
However, this "delicate physiological design" has greatly tested the immunogenicity of the epidemic, so inhalable often needs to be used
with adjuvants.
The choice of adjuvant depends on the type of inhaled vaccine and the inherent immunogenicity of the inhaled vaccine, but because the necessity of adjuvant in inhalable vaccine preparations has not been fully elucidated, the problem of immune response is considered in the study of inhaled vaccines, mainly to investigate the production of immune cells and local antibodies, and to observe whether the adjuvant improves the protective efficacy
of the vaccine.

While the mucosal immune system plays a key role in protecting the respiratory tract from pathogenic invasion, an excessive immune response can damage the lungs and lead to the development
of (antibody-dependent) respiratory diseases.
Some studies have shown that in some cases excessive immune-related responses lead to cytokine storms
.

2 Particle size and deposition

Since the area of infection of the respiratory tract by airborne pathogens depends on the expression of their respective attachment receptors, the sedimentation site of inhaled seedlings is associated with the role of the seedling, so the epidemic preparation should also consider the dispersion within the aerodynamic size range and select the particle size
suitable for penetrating the airway.
Usually 1-5 μm aerosols are suitable for penetration and deposition in the lungs, while smaller particles are easily exhaled
.
Particles larger than 5 μm are mainly deposited in the throat or upper respiratory tract
.
In addition to particle size, other factors such as shape, density, inhalation flow rate, charge, and hygroscopicity may also affect the deposition behavior
of particles in the lungs.

3 The dilemma of molding

A prerequisite for in vivo studies is that the strain should be rigorously evaluated in the animal model closest to humans, which often require the involvement of animal models but are unable to perform the required inhalation actions
.

In addition, the patient's inhalation action is also a factor
affecting the effectiveness of inhalation of the disease.
Patients perform the inhalation action correctly to make the disease reach the desired target area, to maintain the appropriate respiratory flow rate, correct hand breathing coordination, these need to be correct inhalation operation guidance
.
Many trials have considered the use of dry powder inhalation devices (DPIs), which are currently more mature inhalation devices, and refer to dry powder inhalers to optimize the deposition of seedlings in the lungs, while introducing special valves to facilitate the use of
children.

4 Applicability considerations

Inhalable disease preparations can be broadly divided into two categories: liquid preparations and powder preparations
.
Dry powder formulations have several advantages
over liquid formulations.
First of all, properly dried seedlings can be more stable and easy to store in a normal temperature environment, thus avoiding the requirements
for the cold chain.
Second, the weight of dry seedlings is smaller than that of liquid solution, which is conducive to bulk transportation; Third, dry powder formulations can be adapted to disposable dry powder inhalers to prevent reuse, cross-contamination and moisture-induced degradation, and can control the dose
in inhalation compared to liquid formulations.
Such as Kwisha? Mist Superior? It is the use of a nebulizer to atomize the dry powder preparation of the seedlings into tiny particles, through oral inhalation to complete the inoculation
.

LAIV (LAIV) Live attenuated Influenza Vaccine – FluMist, produced by AstraZeneca, was approved by the FDA in 2003 and is the most successful rhinosis vaccine
to date.

FluMist has overcome obstacles such as low nasal mucosal membrane permeability, high mucosal cilia clearance, mucus barrier and enzymatic environment of the nasal mucosa, and has been used in children and adults for more than 10 years
.
The strain contains three attenuated virus vaccines: two A strains (H1N1 and H3N2 subtypes) and one B strain
.
Each virus has the same backbone and consists of six gene fragments from the main donor virus and two gene fragments from wild-type influenza viruses (encoding hemagglutinin and neuraminidase glycoproteins), each LAIV virus is cultured and purified
from eggs.
Genetically engineered to infect and replicate live attenuated viruses in nasopharyngeal mucosal cells, but do not invade the lower respiratory tract and lungs
.
The ability of live attenuated viruses to infect and replicate in the nasopharynx enhances antigen capture and presentation, favoring the development of
immune responses.
In addition, this delivery system allows antigens to be presented in their natural form, and as such, the induced immune response resembles that induced by natural infection
.

The preparation contains some trace impurities, which act as an adjuvant.

All these features and its good security features are the reasons for
FluMist's success.
Then, in February 2012, the FDA approved FluMist tetravalent, a formulation containing two influenza A subtypes and two influenza B viruses, which was also extended to people ages 2 to 49

Of all viral infections and respiratory infections, the only "routinely available" inhaled vaccine is against influenza viruses
.
In the current pandemic, inhalation vaccines are a potential mode of administration, especially dry powder vaccines, which can improve stable and efficient vaccination
.
Compared with injectable vaccines, inhaled vaccines are more widely applicable, more friendly to the elderly, weak and young, and have higher
safety.
In addition, the stronger mucosal immune response induced by inhalation of the disease can provide an additional line of defense, and it is hoped that the inhaled vaccine can enrich the arsenal and fight respiratory diseases
more effectively.

Key references

[1] SUN Xun, ZHANG Yuandong.
Research Progress on Mucosal Disease Delivery Technology to Overcome Immune Barrier[J].
Progress in Pharmacy, 2022,46(4):282-295.

[2] Mato Y L.
Nasal route for vaccine and drug delivery: Features and current opportunities[J].
International Journal of Pharmaceutics, 2019,572:118813.

[3] Heida R, Hinrichs W L, Frijlink H W.
Inhaled vaccine delivery in the combat against respiratory viruses: a 2021 overview of recent developments and implications for COVID-19.
: REVIEW[J].
Expert Review of Vaccines, 2021.

[4] Benne N, Duijn J V, Kuiper J, et al.
Orchestrating immune responses: How size, shape and rigidity affect the immunogenicity of particulate vaccines[J].
Journal of Controlled Release, 2016,234:124-134.