Abstract
The immunological environment of hair follicles has lately received the attention of researchers in the context of transfollicular drug delivery, particularly for improving needle-free transcutaneous immunization. Hair follicles represent shunt pathways across the stratum corneum barrier, which may facilitate the absorption of large or hydrophilic molecules such as vaccine antigens. Currently researchers have identified opportunities and challenges created by transfollicular vaccination. Nanotechnology may facilitate transfollicular delivery in several ways as nanoparticles penetrate deeper and to a higher extent into hair follicles than solutions. Also, nanoencapsulation can stabilize antigens and increase their antigenicity. This seems necessary as only a limited portion of topically applied antigen is available via the hair follicles and as the responsiveness of perifollicular Langerhans cells varies during hair cycle. These problems may be overcome by developing more efficient adjuvant-coupled nanocarriers with high antigen payload.
Hair follicles are a shortcut to circumvent the stratum corneum barrier
One of the main obstacles for any attempt at transdermal delivery is the stratum corneum (SC) barrier. For the purpose of transcutaneous immunization (TCI), that is, needle-free administration of vaccines via the skin Citation[1], neither naked vaccine antigens nor the conventional vaccine formulations can overcome healthy human SC to an extent that would allow the successful delivery of the required dose of antigen. A simple way to circumvent this obstacle is intradermal injection of the vaccine which may be done in a reproducible way by employing special adaptors to standardize injection depth and angle, special injection techniques or a prefilled microsyringe applicator (e.g., Soluvia BD). Other barrier reducing methods (e.g., microneedles, jet injection, dry powder injectors, skin abrasion) are being explored as well. Nonetheless, needle-free vaccination strategies are generally preferred in order to avoid the risk of infection after breaking the SC barrier and the risk of distributing contagious diseases by sharing needles.
TCI combines the advantages of other routes of mucosal vaccination using needle-free strategies being efficient, safe, convenient and economical. In general, mucosal immunization is superior to the conventional intramuscular or subcutaneous injection of vaccines due to the higher number of antigen-presenting cells (APC) which are patrolling the interfaces of the body to the environment in order to build a first line of defense against the invasion of pathogens and thus prevent disease before its onset.
Transfollicular delivery is being explored as an alternative route for vaccine delivery Citation[2–5]. Some experiments hint that transfollicular vaccination may favor a CD8+ biased response which would be promising for developing vaccines against intracellular pathogens, cancer or virus infections Citation[3]. Hair follicles (HF) represent shunt pathways across the SC Citation[6]. In transfollicular vaccination, antigen-loaded nanocarriers shall be trafficked across the HF to reach the large numbers of perifollicular Langerhans cells which may serve as APC Citation[7]. As with glands and other invaginations of the skin, HF contain high numbers of commensal microbes. Therefore, it is not surprising that some special defenses have evolved in order to prevent the invasion of pathogens into the body as well as avoid an overreaction of the immune system to their constant presence. As especially the lower HF has no SC, tight junctions may play a role as compensation mechanisms for the absence of a physical barrier Citation[8]. The tight junctions barrier may interact with the immunological barrier as shown for other epithelia including the interfollicular epidermis Citation[9]. The number as well as the activation state of perifollicular Langerhans cells follows a dynamic pattern which seems to be linked to hair cycling Citation[10]. Also, it was shown that the reaction toward topically applied allergens is less pronounced during anagen (i.e., proliferation) stage Citation[11]. While the reasons for this behavior are unclear, it may be that the success of transfollicular immunization may depend on the developmental state of the HF. As the hair cycle varies between follicles, this may give rise to a higher variability of the effectiveness of transfollicular immunization. To overcome tolerance, it may thus be necessary to use potent antigen, use a formulation with adjuvant properties or synchronize HF cycling and carefully select the time point within the HF cycle when the vaccine is applied. Adjuvants are widely used in vaccine formulations for these purposes and have been used in nanoparticle (NP)-based vaccine formulations with success before Citation[12].
Nanotechnology approaches to transfollicular vaccination
From a drug delivery point of view, vaccine formulations are highly interesting as well as very challenging. Vaccine antigens are complex biological structures, including DNA, peptides, proteins, attenuated viruses, microorganism fragments. Apart from cancer, there is hardly any therapeutic field where such a wide range of colloidal or nanotechnology-based formulations are widely established for the application in humans or animals (e.g., microemulsions, virosomes, ISCOMs and ISCOMATRIX). Alum, which is the oldest and most widely used adjuvant in marketed vaccines, is in effect a colloidal formulation. A large part of the adjuvanticity of alum is based on sustaining the release of an antigen from a gel matrix and thereby controlling its availability and stability. A major reason for the success of sub-micron size particle-based vaccine formulations is certainly that they fall into the size range of common pathogens, where size is a major danger signal and facilitates recognition by the immune system.
Over the past decade, polymeric NPs have emerged as an attractive delivery strategy for antigens and adjuvants. (Nano)encapsulation of antigens and adjuvants can improve stability of biopharmaceuticals and facilitate absorption. In comparison with, for example, virosomes, polymeric NPs may benefit from better designed (often prized as tailor-made) properties such as biodegradability, surface modification and release kinetics. We are still learning which aspects of the carrier design can influence the type of immune response which is elicited. There is indication that size, surface charge and material properties may play a role (for review Citation[13]). These findings are especially noteworthy while considering strategies of immunization against special types of pathogens such as intracellular bacteria, viruses or cancer vaccines where cellular responses are required.
Nanotechnology: size matters
With regards to transfollicular delivery particle size matters for the uptake into the HF. NPs which are applied topically distribute unevenly across the skin surface and stay especially in HF, wrinkles, dermatoglyphs and gland openings. They also penetrate deeper into HF than solutions and form a depot in the follicle that persists for more than 1 week as they are protected from clothing or washout effects Citation[14]. It seems that especially larger particles of roughly 600 nm penetrate deeply into the HF, while larger or smaller NPs remain superficially Citation[15]. Deep penetration is considered important for transfollicular delivery as it carries the antigen-cargo beyond the infundibulum which is still protected by the SC and brings it into the vicinity of the APC.
One of the limiting factors for the success of transfollicular vaccination is the efficient follicular penetration of the nanocarriers. First results show that the overall penetration into the HF is probably less than 10% of the total applied amount, with the remaining 90% being deposited mostly on the skin surface where they cannot reach the APC and are not therapeutically available Citation[2]. Currently, it is not clear whether other factors than size, such as NP material or charge may influence the uptake into HF. This knowledge is required for optimizing transfollicular vaccine formulations.
Conclusion
Vaccination strategies with higher efficiency are sorely needed, especially to cope with the needs of immunosuppressed and elderly patients or with the prospect of dose sparing to enable protection of a higher number of people in the case of pandemics. Factors determining the efficiency of a vaccine besides the route of administration include the type of antigen that is used, whether or not the vaccine contains an adjuvant, which adjuvant this is and also how the vaccine is formulated. These factors will play a critical role in the type of immune response that is elicited (type 1 cellular, type 2 humoral response) so that selecting the right kind of strategy will be tantamount to success when fighting against different types of pathogens or malignancies.
Particle-based vaccines hold great promise for TCI. Transfollicular delivery is one of many strategies which is currently explored. It is a major advantage of transfollicular immunization that an immnune response can be evoked while leaving the SC barrier intact. Furthermore, it seems that it is feasible to elicit cellular immune responses via this delivery route. In order to advance transfollicular delivery beyond the experimental stage, solutions will have to be found for the sub-optimal penetration efficiency into the HF, for designing efficient and safe carrier–antigen–adjuvant combinations and developing simple, reproducible and patient friendly ways for topical application of these kinds of vaccines.
Financial & competing interests disclosure
CM Lehr is Professor at Saarland University and CEO of its subsidiary, Pharmbiotec Gmbh, offering pharmaceutical R&D services. S Hansen is acting as consultant to the same company. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending or royalties.
No writing assistance was utilized in the production of this manuscript.
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