TCR repertoire analysis after Hepatitis A vaccination

Ageing is generally associated with dysfunction in both the adaptive and the
innate immune system, which renders the elderly population more susceptible to
infectious diseases. Additionally, reduced vaccine response is observed in the
elderly, which limits preventative care. The ability to induce protection with
the influenza vaccine, for example, declines with age, with an efficacy between
70% and 90% in children and adults, dropping to 30-50% for those over 65 years
of age. Together, the declining immune function and reduced vaccine efficacy
leave the elderly susceptible to infectious diseases. One amongst the many
effects of ageing on the immune system is the compromised ability of T cells to
respond to novel infections and manage chronic infections. It is generally
regarded that these decreased immune responses in the elderly are due to
reduced T-cell diversity at older age owing to many factors, such as thymic
involution and peripheral clonal expansion of antigen-specific T cells.

The ability to mount a T cell response to new antigens is dependent on T cell
frequency and functionality and T cell receptor (TCR) diversity. TCRs are
highly polymorphic heterodimers which are responsible for the recognition of
antigens. The diversity of the TCRs recruited in response to an infection is
directly related to the outcome of the infection wherein low diversity TCR
repertoires allow for more rapid viral escape.

Many studies have demonstrated a loss of TCR repertoire diversity with age.
Olga et al showed that with age there is a loss of diversity in the T cell pool
and that the more expanded clones make up an increasing fraction of the T-cell
pool [6]. This is somehow expected, as with age i) the thymus, which is
responsible for the generation of T cells with new specificities, undergoes
involution, and ii) the number of infections an individual has undergone
increases. Hence, the number of clonal expansions in the T-cell repertoire
increases, resulting in an overall loss of TCR diversity. More specifically in
the naïve T-cell pool, a study has shown that with age, high frequency clones
(T cells with the same TCRβ chain) in the naive CD4 T cell pool make up a
larger fraction of the total TCRβ diversity, suggestive for a reduced diversity
in the naïve TCR repertoire.

These changes in the T-cell pool are generally held responsible for the
impaired immune responses that are typically observed in older individuals. The
idea is that due to *holes* in the naïve T cell repertoire, older individuals
have an impaired capacity to respond to new antigens. Indeed, a study conducted
in old mice demonstrated restricted TCR repertoire diversity of the
epitope-specific T cell pool in an influenza model. The epitope-specific T
cells that were present in low frequencies in the naive T-cell compartment of
younger mice were found to be less frequent at older age [8]. It remains
unknown whether a similar effect occurs in humans. It is imperative to realize
that conclusions from mouse models cannot be directly applied to humans as the
size of the T cell compartment and the mechanisms of maintenance of T cells
differ between mice and men. This motivates a separate investigation in humans.

In this study we aim to answer the question whether TCR diversity becomes
limiting at older age and thereby leads to impaired responses to novel
antigens. While many studies have suggested a contraction in TCR diversity with
age, a one-to-one comparison between the TCR diversity between young and old
age groups, in response to a new antigenic stimulus (here, vaccine) has not
been made. This study design will help us to study the difference in the TCRs
recruited from the naïve T cell pool in response to an antigenic stimulus and
will thus provide insight into the functional loss of TCR diversity with age.

This study is divided into two parts- Phase I and Phase II.


The first part of the study is called Phase I is an observational study with
invasive procedure in the form of blood draws.

In Phase I, we will recruit participants previously vaccinated with Hepatitis A
to determine the frequency of Hepatitis A specific T cells, and the blood
volume required to sort these cells in sufficient numbers for TCR repertoire
analysis. We also want to quantify the frequency of cells that are
cross-reactive/falsely reactive to Hepatitis A. Therefore, we will also recruit
healthy adults naïve to Hepatitis A ( i.e. no previous vaccination or infection
record of Hepatitis A). Recruiting participants who have been previously
vaccinated allows us to reduce number of participants receiving a vaccine as an
intervention in our study.

In Phase II, we will address the main objective of this study.

To this end, we will recruit a cohort of Young (18-30 years) and Older (65-80
years) healthy individuals naïve to Hepatitis A. CMV and Hepatitis A naïve
individuals will be vaccinated with two doses of VAQTA Adult® vaccine and their
Hepatitis A specific TCR repertoire will be analysed.

Invasive procedures: In Phase I, maximally two blood samples and in Phase II,
maximally five blood samples will be taken from all participants. In addition,
with each visit, participants are asked to fill in a short questionnaire about
their health.

Interventions in this study: 20 (10 young and 10 older) participants from Phase
II will be vaccinated with two doses of the VAQTA Adult® vaccine which is a
vaccine against Hepatitis A virus.
Phase II can be described as an observational, cross sectional study with an
intervention and invasive procedures.

For this research it is essential that we follow the antigen-specific T-cell
response to a pathogen that participants have not encountered before and do not
encounter during the study. Hepatitis A is a low incidence disease in the
Netherlands with an average of 142 cases per year between 2013 and 2020 11.
Vaccination against Hepatitis A is not mandatory in the Netherlands but is
recommended as a travel vaccine for certain countries. Thus, we regard it
feasible to recruit enough participants who are immunologically naive to
Hepatitis A, who can receive the vaccination for the first time. This logic
dictates the choice of vaccination in this study.

Here, we use VAQTA as a tool to induce an immune response to a new antigen in
Phase II participants. We are not addressing the *diagnostic, prophylactic or
therapeutic potential of the medicinal product nor its pharmacokinetic or
pharmacodynamic profile* as defined by Regulation (EU) No 536/2014. VAQTA
Adult® already has marketing authorization in the EU, for both age groups, and
we use it as a non-investigational product that is used as a means to follow
antigen-specific T cells, in order to study the effect of ageing on the immune
response.

20 Participants of Phase II will receive 2 doses of the VAQTA Adult® vaccine
against Hepatitis A virus. The VAQTA Adult® vaccine is regarded safe for both
age groups and could possibly benefit the health of vaccinees by providing
protection against Hepatitis A. Participants in Phase I will donate blood a
maximum of two times, while participants in Phase II will donate blood five
times maximally. The risks of this study are minimal as VAQTA is an EMA/FDA
approved vaccine and the main burden on the participants is to receive the two
doses of vaccine, the blood draws and the time consumed for the visits to the
hospital.