Inducing high magnitude of antibodies with epitope breadth over prolonged periods of time is likely a
prerequisite to prevent several of the world’s most serious infectious diseases such as HIV-1, malaria
and tuberculosis for which there are no vaccines yet. A much better understanding of the innate
immune mechanisms that are critical for inducing strong responses to vaccination is therefore
essential. The overall aim of this thesis was to characterize early innate immune responses in vivo
after administration of antigens. This includes studies of the recruitment of immune cell subsets to the
site of antigen injection (e.g. skin or muscle), local cell activation and presence of inflammatory
mediators, antigen uptake and transport and finally initiation of adaptive immunity in lymph nodes
(LNs). To be able to approach this in humans in vivo, we first utilized skin punch biopsies collected
from sites injected with purified protein derivate (PPD), which is a mixture of mycobacterial antigens
used in the tuberculin skin test (TST). By performing tissue staining of cryosections we show in paper
I that several subsets of dendritic cells (DCs), including the plasmacytoid DCs (PDCs) normally not
residing in skin, infiltrated the dermis in the positive TST indurations, which was in contrast to donormatched
saline-injected skin. The positive TST indurations were associated with cell death and high
expression of the antimicrobial peptide LL37, which together can provide means for PDC activation
and IFNα production. In line with this, IFN-inducible MxA was highly expressed in the positive TST
sites. We expanded the studies in paper II and DC accumulation was also found in skin biopsies taken
after skin tests using antigens from either mumps virus or Candida albicans. Further, TST indurations
of HIV-1+ individuals also showed DC infiltration but to a lower degree, which likely reflect on the
reduced integrity of their immune system. To this end, the level of DCs in the positive TST reactions
correlated with the level of infiltrating T cells.
The skin antigen tests represent recall of immunological memory responses locally. To enable studies
of local innate immune activation after vaccine administration and priming of naïve responses, we
developed a nonhuman primate (NHP) model in the second part of the thesis. After establishing and
validating protocols for sample collection and tissue processing in paper III, we examined in paper IV
how the distinct vaccine adjuvants; alum (benchmark), MF59 (emulsion) or alum with TLR7 agonist,
influence the innate responses leading to adaptive immunity. HIV-1 envelope glycoprotein (Env) was
administered as the vaccine antigen together with the adjuvants. We found a rapid infiltration of
neutrophils, monocytes and DCs to the vaccine-injected muscle with all adjuvants. Env+ cells were
readily detected in the muscle and draining LNs. In line with the finding of alum-TLR7 and MF59
being superior over alum in terms of inducing both antibody- and T cell responses, alum consistently
showed lower innate activation. While alum-TLR7 consistently induced robust DC maturation and
type IFN I responses, MF59 induced neutrophil homing to LNs. Comparison of antigen presentation
capacity of Env+ cells in the draining lymph nodes showed that myeloid DCs exceled at stimulating
Env-specific CD4+ T cell responses. However, neutrophils were also capable of antigen presentation.
Despite inducing different innate activation, both MF59 and alum-TLR7 enhanced the priming of
Env-specific T cells in vaccine-draining LNs as well as increased the differentiation of T follicular
helper cells and germinal center formations compared to alum. In summary, our findings demonstrate
the initial immune events at the sites of antigen delivery, including vaccination in vivo. These early
immunological responses shape the quantity and quality of adaptive immunity. Understanding the
mechanisms by which distinct adjuvants influence vaccine response will help in the selection of the
best-suited adjuvant to improve vaccine efficacy to a given pathogen
