br Introduction Cell surface receptors are central to the

Introduction Cell surface receptors are central to the interaction of cells with their surroundings and play an important role in determining functional responses. These receptors are integral membrane proteins responsible for the binding of extracellular signaling molecules and transduction into intracellular signaling molecules, which change cellular behavior. These proteins perform important functions in cell signaling, motility, adhesion, cell-cell interaction, and antigen recognition. As extracellular signaling molecules bind to cell membrane receptors, a cascade of interactions occurs leading to conformational and chemical changes (Deller and Yvonne Jones, 2000, Wagner and Wyss, 1994, Kirkbride et al., 2005). Many of these interactions are also associated with pathogen infections, and therefore, understanding the mechanism of these ligand-receptor interactions may have profound implications in physiology and medicine. One important membrane receptor is CD4, a 55kDa glycoprotein with four immunoglobulin (Ig)-like domains (D1–D4) expressed in T lymphocytes (Wang et al., 1990, Ryu et al., 1990, Matthias et al., 2002, Maddon et al., 1985). CD4 assists T cell receptor in communicating with an antigen presenting cell through an interaction with the major histocompatibility complex class II molecules. This interaction enhances the signal transduction by the interaction between CD4's cytoplasmic tail and lymphocyte-specific protein tyrosine kinase (Veillette et al., 1988, Veillette et al., 1989, Weiss and Littman, 1994) which ultimately leads to T cell proliferation and differentiation (Doyle and Strominger, 1987, Parnes, 1989). CD4 is also known to be a preliminary receptor for HIV-1 infection (Dalgleish et al., 1984). The viral envelope glycoprotein gp120 binds to D1 to help HIV-1 to gain entry into cell (Dalgleish et al., 1984, Sattentau et al., 1989, Healey et al., 1990). Upon binding, CD4 triggers a change in gp120 structure that promotes recognition of the chemokine receptors and ultimately leads to membrane fusion (Wu et al., 1996, Zhou et al., 2007). Although it is well known that some GYKI 52466 dihydrochloride have capability to restrict HIV-1 infection (Sattentau et al., 1986, Song et al., 2010, Moir et al., 1999, Helling et al., 2015), the mechanism behind this remains imperfectly defined. Here we solve the problem by measuring binding strength, thermodynamics, kinetics, and assembly state of the interactions between recombinant human CD4 and anti-human CD4 monoclonal antibodies (mAb), Leu3a and OKT4, using atomic force microscopy (AFM), isothermal titration calorimetry (ITC), and circular dichroism spectroscopy. Leu3a binds to D1 (within the first 79 amino acids) (Healey et al., 1990) and has capability to block HIV-1 entry (HIV-1 binds to residues 41–52) (Sattentau et al., 1989) while OKT4 binds to D3 and cannot block HIV-1 entry (Sattentau et al., 1986, Moir et al., 1999).
Materials and methods
Results and discussion
Conflict of interests
Acknowledgements This work was supported by grants from the German Federal Ministry of Education and Research (BMBF, FKZ 03Z2CN11 and FKZ 03Z2CN12) and from German Research Foundation (DFG, NG 133/1-1). We thank M. Hundt, H.P. Müller, and M. Delcea for supports and all ZIK HIKE lab members for advice and fruitful discussions.
Introduction Protection against disease requires immune system detection and response to a diverse array of episodic pathogenic insults that attack the host at unpredictable times and varied locations. To deal with this “random” yet omnipresent potential for infection, the adaptive immune system relies on antigen-specific lymphocytes that circulate systemically. Estimates of naive precursor frequency suggest the number of T cells specific for a given foreign antigen is very low, in the range of ten to a few hundred cells per mouse (Jenkins and Moon, 2012), with comparable scaling in humans (Jenkins and Moon, 2012, Yu et al., 2015). Given this low frequency, the vast space that T cells must survey, and the limiting number of cognate antigen-presenting cells (dendritic cells [DCs]) early after infection, it seems improbable for a rare T cell to find its ligand efficiently unless positional cues and directed search strategies are in place to facilitate such detection (Qi et al., 2014).