Enhancing Antigen Cross-Presentation in Human Monocyte-Derived Dendritic Cells by Recruiting the Intracellular Fc Receptor TRIM21

Suboptimal immune responses to pathogens contribute to chronic infections. One way to improve immune responses is to boost Ag presentation. In this study, we investigate the potential of the tripartite motif-containing 21 (TRIM21) pathway. TRIM21 is a ubiquitously expressed cytosolic protein that recognizes the Fc region of Abs. When Abs that are bound to pathogens enter the cell as immune complexes, binding of TRIM21 to Fc initiates downstream inflammatory signaling and targets the immune complexes for proteasomal degradation. In APCs, peptides generated by proteasomes are loaded onto MHC class I molecules to stimulate CD8 T cell responses, which are crucial for effective immunity to pathogens. We hypothesized that increasing the affinity between immune complexes and TRIM21 might markedly improve CD8 T cell responses to Ags processed by the TRIM21 pathway. Using phage display technology, we engineered the human IgG1 Fc to increase its affinity for TRIM21 by 100-fold. Adenovirus immune complexes with the engineered Fc induced greater maturation of human dendritic cells (DC) than immune complexes with unmodified Fc and stimulated increased Ag-specific CD8 T cell proliferation and IFN-γ release in cocultures of DC-PBMC. Thus, by increasing the affinity between Fc and TRIM21, Ags from immune complexes undergo enhanced cross-presentation on DC, leading to greater CD8 T cell responses. Our study reveals an approach that could potentially be used in vaccines to increase cytotoxic T cell responses against Ags that are targeted or delivered by Fc-modified Abs.

C-reactive protein collaborates with plasma lectins to boost immune response against bacteria

Although human C-reactive protein (CRP) becomes upregulated during septicemia, its role remains unclear, since purified CRP showed no binding to many common pathogens. Contrary to previous findings, we show that purified human CRP (hCRP) binds to Salmonella enterica, and that binding is enhanced in the presence of plasma factors. In the horseshoe crab, Carcinoscorpius rotundicauda, CRP is a major hemolymph protein. Incubation of hemolymph with a range of bacteria resulted in CRP binding to all the bacteria tested. Lipopolysaccharide-affinity chromatography of the hemolymph co-purified CRP, galactose-binding protein (GBP) and carcinolectin-5 (CL5). Yeast two-hybrid and pull-down assays suggested that these pattern recognition receptors (PRRs) form pathogen recognition complexes. We show the conservation of PRR crosstalk in humans, whereby hCRP interacts with ficolin (CL5 homologue). This interaction stabilizes CRP binding to bacteria and activates the lectin-mediated complement pathway. We propose that CRP does not act alone but collaborates with other plasma PRRs to form stable pathogen recognition complexes when targeting a wide range of bacteria for destruction.

Diversity in lectins enables immune recognition and differentiation of wide spectrum of pathogens

Carbohydrate-binding lectins play essential roles as pattern recognition receptors in innate immunity in both vertebrates and invertebrates. The carcinolectins 5 (CL5a and CL5b, the CL5 isoforms of horseshoe crab, Carcinoscorpius rotundicauda, with apparent sizes of 36 and 40 kDa, respectively) are prominent plasma lectins that bind all representative microbes and pathogen-associated molecular pattern molecules. Different cDNA isoforms of both CL5a and CL5b were isolated, leading to our speculation on their functional divergence. Characterization of CL5 isoforms bound to microbial cell surfaces demonstrates the diversity of these lectins. The resolution patterns of the isoforms that associate with fungus differ from those that associate with bacteria, suggesting the unique roles these lectins play in the recognition and differentiation of microbes. We postulate that different populations of plasma lectins act in collaboration in frontline innate immune defense against disparate pathogens. The functional diversity of lectins in invertebrates appears to evolutionarily compensate for the lack of acquired immunity.

The antimicrobial properties of C-reactive protein (CRP)

C-reactive protein, CRP, is a predominant pattern-recognition receptor (PRR) in the plasma of the horseshoe crab, which recognizes lipopolysaccharide (LPS). Native CRP2 has previously been shown to exhibit agglutination activity against the polysialic capsule of Escherichia coli K1 but its role in bacterial clearance is not well characterized. In this work, the antimicrobial activity of a recombinant CRP2 isoform (rCRP2) was tested against E. coli, Pseudomonas aeruginosa and Staphylococcus aureus. rCRP2 agglutinates bacteria and exhibits bactericidal activity against Gram-negative bacteria. In addition, the antimicrobial activity of rCRP2 is calcium-independent. GST pulldown experiments suggest that in the naïve physiological state, CRP2 interacts with hemocyanin, native CRPs, a 35-kDa plasma lectin and an as yet unidentified 40-kDa protein. This interaction was enhanced upon Pseudomonas infection. We propose that rCRP2 is a PRR with potent antimicrobial activity and its interacting partners contribute to effective bacterial clearance.

C-reactive protein: a predominant LPS-binding acute phase protein responsive to Pseudomonas infection

As a structural component of the outer membrane of Gram-negative bacteria, endotoxin, also known as lipopolysaccharide (LPS) exhibits strong immunostimulatory properties, rendering it a pivotal role in the pathogenesis of Gram-negative septicaemia. Our attempt to identify LPS-binding proteins from the hemolymph of the horseshoe crab led to the isolation and identification of Creactive protein (CRP) as the predominant LPS-recognition protein during Pseudomonas infection. CRP is an evolutionarily ancient member of a superfamily of 'pentraxins'. It is a major protein in acute phase of infection in humans. Our investigation of CRP response to Pseudomonas aeruginosa unveiled a robust innate immune system in the horseshoe crab, which displays rapid suppression of a dosage of 10(6) CFU of bacteria in the first hour of infection and effected complete clearance of the pathogen by 3 days. Such a high dose would have been lethal to mice. Full-length CRP cDNA was cloned. Analysis of the untranslated regions suggests their crucial role in post-transcriptional regulation of CRP transcript levels. Northern blot analysis demonstrated an acute up-regulation of CRP by about 60-fold in 6-48 h of Pseudomonas infection. Taken together, our results provide new insights into the importance of CRP as a conserved molecule for pathogen recognition.