K. Christopher Garcia

Garcia's earliest research as a graduate student at Johns Hopkins University focused on understanding how anti-idiotyopic antibodies recognize peptide antigens.[10] As a postdoctoral scholar at The Scripps Research Institute, Garcia conducted a groundbreaking study that revealed how T cells of the immune system survey peptides presented by major histocompatibility complex proteins (MHC), thus allowing them to distinguish between "self" and "non-self". Garcia's research led to the first visualization of a T cell receptor (TCR) bound to a peptide-MHC (pMHC) complex and was published in the journal Science in 1996.[11] Garcia's 1996 article on the TCR-MHC interaction has had broad impact in the fields of immunology and immunotherapy.[12]

At Stanford University, the Garcia Laboratory reported the structure of the pre-B cell receptor (pre-BCR) in 2007, which revealed how pre-BCRs oligomerize to signal in the absence of antigen.[13] Garcia's group has also authored several additional landmark articles exploring various aspects of TCR-pMHC interactions, including the first structure of a γδ TCR-pMHC complex,[14] the molecular basis for dual recognition of "self" and "foreign" MHCs by TCRs,[15] insights into the germline basis of TCR/MHC interactions,[16][17] the extent of cross-reactivity in the TCR repertoire,[18][19] and elucidation of the structural trigger for TCR signaling.[20] In Garcia's most recent work, his lab developed a peptide-MHC library technology that has enabled the discovery of antigens for orphan T cell receptors, such as those resident in tumors. This technology also enabled a breakthrough in understanding how signaling is intitiated by pMHC engagement.

Garcia's research has established how structural and biophysical principles govern receptor binding and signal activation in many different cytokine systems. Key findings include determination of the first crystal structures of the following cytokine family members in complex with their surface receptors: gp130 family (IL-6),[21] common gamma (γc) family (IL-2),[22] Type I Interferons (IFNα2/IFNω)[23] and Type III Interferons.[24] The Garcia Laboratory has also determined crystal structures of many other major cytokine-receptor complexes including those of IL-1, IL-4, IL-13, IL-15, IL-17, IL-23, LIF and CNTF. These structures have revealed a wide range of binding topologies and architectures, and demonstrate how convergent evolution has provided many solutions for cytokine receptors to transduce signals across the cell membrane. In addition to molecular studies of cytokines, Garcia's group has also used directed evolution to engineer high affinity cytokine variants (IL-2, IL-4, IFN-λ) with improved therapeutic properties.[25][24][26]

In 2012, Garcia's laboratory determined the crystal structure of a Wnt protein in complex with its cellular receptor, Frizzled.[27] The Wnt-Frizzled structure indicated that Wnts utilize a post-translational lipid modification to directly engage the Frizzled extracellular domain, which represents a highly unusual binding mode among soluble ligands. Garcia's study revealed a striking, donut-shaped architecture adopted by the Wnt-Frizzled complex that adorns the cover of the July 6th, 2012 issue of Science.[27] More recently, Garcia's laboratory reported a breakthrough in being able to recapitulate canonical Wnt signaling using water-soluble bispecific ligands that dimerize Frizzled and Lrp6, which has important implications for the development of therapeutics for regenerative medicine.[28]

In 2015 and 2017, Garcia published articles in Science describing the first atomic-level visualizations of Notch signaling complexes.[29][30] Garcia's group used directed evolution to strengthen low-affinity interactions between the receptor Notch1 and ligands Delta-like 4 (DLL4) and Jagged1 (Jag1) as a means of stabilizing the complexes for co-crystallization. Notch1-DLL4 and Notch1-Jag1 structures were determined by x-ray crystallography and revealed long, narrow binding interfaces assisted by multiple O-linked fucose and glucose modifications on Notch1. O-linked glycans are rarely observed at protein-protein interfaces, and their presence at the Notch-ligand interface explained how changes in glycosylation state influence Notch signaling activity. Garcia's 2017 publication also established that Notch-ligand interactions form catch bonds, and that Delta-like and Jagged ligands have different mechanical force thresholds for Notch receptor activation.[30]

In 2015, the Garcia Laboratory reported the x-ray crystal structure of the virally encoded G-protein coupled receptor (GPCR), US28, bound to its chemokine ligand, fractalkine (CX3CL1).[31] The US28-Fractalkine structure was one of the first reports to visualize a protein ligand bound to a GPCR, and revealed that the globular "head" of fractalkine docks onto the extrcellular loops of US28, while fractalkine's flexible N-terminal "tail" threads into a cavity in the center of US28 as a means of fine-tuning its downstream signaling activity. In more recent studies, the lab has engineered biased chemokine ligands and shown that GPCR activation is governed by ligands that induce shape changes rather than highly specific bonding chemistries.[32]

Garcia has conducted several studies targeting cellular receptors for applications in cancer immunotherapy. In 2013, Garcia's group developed high affinity antagonists of the receptor CD47 that potently enhance the antitumor effects of established therapeutic antibodies.[33] Garcia later determined that the therapeutic effects of CD47 blockade require combination therapy with checkpoint blockade antibodies in immunocompetent hosts, thus proving that CD47-based therapy relies upon stimulation of the adaptive immune system.[34] Garcia's lab published the creation of an "orthogonal" IL-2 receptor complex to enable the selective delivery of IL-2 signals to engineered T cells during adoptive cell therapy.[35] They also reported a new technology using yeast-displayed peptide-MHC molecules to identify tumor antigens recognized by Tumor Infiltrating Lymphocytes.[36]