For an immune response to occur, T lymphocytes must be exposed to two types of stimulus. Signal no.1, the antigen, ensures the specificity of activation and is transduced via the T cell antigen receptor/CD3 complex. Signals no.2 for T cells are co-stimulators such as CD28 that together with signal no.1 promote expression or upregulation of cytokines and receptors such as interleukin-2 and its receptor CD25 for clonal expansion of the specific T cells and their differentiation into effector and memory cells. In addition to this positive branch, the immune system provides for control mechanisms, which maintain homeostasis after active immune responses to foreign antigens, and prevent or abort responses to self-antigens. In this respect, there are also neo-expressed and up-regulated signal no.3 molecules, which act as inhibitors and contribute to the active downregulation of T cell responses. Molecules involved for counterbalancing the immune response appear to include for instance the cytokines interleukin-10, TGF-beta, and the receptors CTLA-4 and PD-1.
We consider that understanding of the array of positive and negative co-signals of T cells will give the insight how the immune system distinguishes between self and non-self, between danger and normal, what has to be tolerated and what to be attacked. These positive and negative co-signals are therefore potential therapeutic targets to treat diseases caused by aberrant T cell reactions found e.g. in autoimmune diseases or allergy, but they might also be useful as natural adjuvants to augment the immune responses in e.g. vaccination. Therefore, we have been searching in the last years for such molecules and identified several of them including T-CD31L, CD92 and CD147. Currently, we are analyzing the molecular mechanisms underlying regulation of T cells via these receptors and, in particular, how lipid-rafts are regulating the underlying pathways. Particular efforts we put on high-throughput and ultra-sensitive imaging systems to analyze spatially- and temporarily-resolved in the living cells the function of key signaling molecules, e.g. the Src protein tyrosine kinase Lck.
GPI-anchored proteins (GPI-proteins) are a family of cell surface receptors, which upon post-translational modification become covalently attached to the glycolipid glycosylphosphatidylinositol (GPI) that serves as anchor of these molecules in the outer leaflet of the plasma membrane. Thus, these proteins are devoid of transmembrane and cytoplasmic domains and, therefore, it is a mystery how these proteins can transmit signals across the membrane. This information, however, is crucial because a number of critical receptors belong to this family, e.g. CD14, the receptor for the bacterial cell wall component lipopolysaccharide; CD16b, the IgG-Fc receptor type 3 on granulocytes; the complement and T cell regulatory proteins CD55 and CD59; CD87, the urokinase plasminogen activator receptor, a multifunctional molecule in adhesion and migration of cells. The importance of this class of molecules in immune cell (leukocyte) biology is manifested in paroxysmal nocturnal hemoglobinuria patients (specifically their leukocytes lack GPI-proteins due to a processing defect in the hematopoietic stem cell). Beside abnormal sensitivity of erythrocytes towards complement, these patients suffer on thrombosis and have serious problems to fight against infections.
In collaboration with several national and international research groups we could show that GPI-proteins are associated in special plasma membrane compartments, called GPI-microdomains or lipid-rafts, with members of the Src protein tyrosine kinase family, key molecules in signal transduction and cell activation. Furthermore, we provided evidence that lipid-rafts serve GPI-proteins to meet other leukocyte receptors, which serve them as transmembrane adaptors for function and signal transduction across the membrane. During the last decade there was a big debate about the existence of lipid-rafts. However, today they are more and more believed to function as platforms for controlling initiation of signal transduction of cell surface receptors at and across the plasma membrane. Currently, in collaboration with national and international partners we are establishing novel biochemical and microscopic techniques to analyze heterogeneity, fine structure and organization of lipid-rafts in living immune cells.