Knocking at the brain’s door: intravital two-photon imaging of autoreactive T cell interactions with CNS structures

Since the first applications of two-photon microscopy in immunology 10 years ago, the number of studies using this advanced technology has increased dramatically. The two-photon microscope allows long-term visualization of cell motility in the living tissue with minimal phototoxicity. Using this technique, we examined brain autoantigen-specific T cell behavior in experimental autoimmune encephalitomyelitis, the animal model of human multiple sclerosis. Even before disease symptoms appear, the autoreactive T cells arrive at their target organ. There they crawl along the intraluminal surface of central nervous system (CNS) blood vessels before they extravasate. In the perivascular environment, the T cells meet phagocytes that present autoantigens. This contact activates the T cells to penetrate deep into the CNS parenchyma, where the infiltrated T cells again can find antigen, be further activated, and produce cytokines, resulting in massive immune cell recruitment and clinical disease

Modelling the regulation of telomere length: the effects of telomerase and G-quadruplex stabilising drugs

Telomeres are guanine-rich sequences at the end of chromosomes which shorten during each replication event and trigger cell cycle arrest and/or controlled death (apoptosis) when reaching a threshold length. The enzyme telomerase replenishes the ends of telomeres and thus prolongs the life span of cells, but also causes cellular immortalisation in human cancer. G-quadruplex (G4) stabilising drugs are a potential anticancer treatment which work by changing the molecular structure of telomeres to inhibit the activity of telomerase. We investigate the dynamics of telomere length in different conformational states, namely t-loops, G-quadruplex structures and those being elongated by telomerase. By formulating deterministic differential equation models we study the effects of various levels of both telomerase and concentrations of a G4-stabilising drug on the distribution of telomere lengths, and analyse how these effects evolve over large numbers of cell generations. As well as calculating numerical solutions, we use quasicontinuum methods to approximate the behaviour of the system over time, and predict the shape of the telomere length distribution. We find those telomerase and G4-concentrations where telomere length maintenance is successfully regulated. Excessively high levels of telomerase lead to continuous telomere lengthening, whereas large concentrations of the drug lead to progressive telomere erosion. Furthermore, our models predict a positively skewed distribution of telomere lengths, that is, telomeres accumulate over lengths shorter than the mean telomere length at equilibrium. Our model results for telomere length distributions of telomerase-positive cells in drug-free assays are in good agreement with the limited amount of experimental data available

An autoimmunity odyssey: how autoreactive T cells infiltrate into the CNS

Experimental autoimmune encephalomyelitis (EAE) is a widely used animal model of multiple sclerosis (MS), a human autoimmune disease. To explore how EAE and ultimately MS is induced, autoantigen-specific T cells were established, were labeled with fluorescent protein by retroviral gene transfer, and were tracked in vivo after adoptive transfer. Intravital imaging with two-photon microscopy was used to record the entire entry process of autoreactive T cells into the CNS: a small number of T cells first appear in the CNS leptomeninges before onset of EAE, and crawl on the intraluminal surface of blood vessels, which is integrin a4 and aL dependent. After extravasation, the T cells continue into the perivascular space, meeting local antigen-presenting cells (APCs), which present endogenous antigens. This interaction activates the T cells and guides them to penetrate the CNS parenchyma. As the local APCs in the CNS are not saturated with endogenous antigens, exogenous antigens stimulate the autoreactive T cells more strongly and, as a result, exacerbate the clinical outcome. Currently, we are attempting to visualize T-cell activation in vivo in both rat T-cell-mediated EAE and mouse spontaneous EAE models

Imaging of immune cell behavior and function in multiple sclerosis and experimental autoimmune encephalomyelitis

To visualize the entire process of encephalitogenic T cell infiltration into the target organ, we performed intravital imaging by using two-photon microscopy in experimental autoimmune encephalomyelitis, the animal model of multiple sclerosis. Intravital imaging documented that T cells first appear in the leptomeningeal blood vessels where they crawl in an integrin-dependent manner and scan the intraluminal surface for extravasation sites. After diapedesis, the T cells continue to crawl on the abluminal surface, where they meet local antigen presenting cells (APC) that can provide stimuli to the T cells for the subsequent infiltration into the central nervous system (CNS) parenchyma. Although flow cytometric analysis documented that the infiltrated T cells upregulated their activation markers in the CNS meninges, it was unclear at which scanning step the activation occurred. We recently introduced two genetically encoded fluorescent T cell activation sensors for intravital imaging. The first is a fluorescent resonance energy transfer-based Ca2+ sensor for the quantification of the intracellular Ca2+ concentration, a major step in T cell receptor signaling. The second sensor is a truncated nuclear factor of activated T cells fused to green fluorescent protein, which subcellular localization corresponds to the T cell activation state. Introducing these sensors into the lymphocytes enables the visualization of the interactions of encephalitogenic T cells with different blood–brain barrier structures, and allows us to assess the functional aspect of these interactions directly in vivo. This model system can be further used to evaluate therapeutic compounds and to better understand the activities of T cells in vivo