Sterilizing immunity: Understanding COVID-19.

Immune memory develops during primary infections to protect from future exposures to the same pathogen. Vaccines mimic this response and induce immune memory that protects from severe disease and, in some cases, from symptomatic infection. If the pathogen is eliminated before it can replicate, natural and vaccine-induced immune memory can prevent the establishment of the infection, mediating sterilizing immunity. Sterilizing immunity protects the individual and prevents transmission to new hosts, thereby contributing to protection at a population level. Here, we describe the basic concepts of sterilizing immunity and discuss its relevance for protection in the context of SARS-CoV-2.

Clonal evolution and TCR specificity of the human TFH cell response to Plasmodium falciparum CSP.

T follicular helper (TFH) cells play a crucial role in the development of long-lived, high-quality B cell responses after infection and vaccination. However, little is known about how antigen-specific TFH cells clonally evolve in response to complex pathogens and what guides the targeting of different epitopes. Here, we assessed the cell phenotype, clonal dynamics, and T cell receptor (TCR) specificity of human circulating TFH (cTFH) cells during successive malaria immunizations with radiation-attenuated Plasmodium falciparum (Pf) sporozoites. Repeated parasite exposures induced a dynamic, polyclonal cTFH response with high frequency of cells specific to a small number of epitopes in Pf circumsporozoite protein (PfCSP), the primary sporozoite surface protein and well-defined vaccine target. Human leukocyte antigen (HLA) restrictions and differences in TCR generation probability were associated with differences in the epitope targeting frequency and indicated the potential of amino acids 311 to 333 in the Th2R/T* region as a T cell supertope. But most of vaccine-induced anti-amino acid 311 to 333 TCRs, including convergent TCRs with high sequence similarity, failed to tolerate natural polymorphisms in their target peptide sequence, thus demonstrating that the TFH cell response was limited to the vaccine strain. These data suggest that the high parasite diversity in endemic areas will limit boosting of the vaccine-induced TFH cell response by natural infections. Our findings may guide the further design of PfCSP-based malaria vaccines able to induce potent T helper cell responses for broad, long-lasting antibody responses.

How to induce protective humoral immunity against Plasmodium falciparum circumsporozoite protein.

The induction of protective humoral immune responses against sporozoite surface proteins of the human parasite Plasmodium falciparum (Pf) is a prime goal in the development of a preerythrocytic malaria vaccine. The most promising antibody target is circumsporozoite protein (CSP). Although PfCSP induces strong humoral immune responses upon vaccination, vaccine efficacy is overall limited and not durable. Here, we review recent efforts to gain a better molecular and cellular understanding of anti-PfCSP B cell responses in humans and discuss ways to overcome limitations in the induction of stable titers of high-affinity antibodies that might help to increase vaccine efficacy and promote long-lived protection.

An efficient single-cell based method for linking human T cell phenotype to T cell receptor sequence and specificity.

Single-cell antigen-receptor gene amplification and sequencing platforms have been used to characterize T cell receptor (TCR) repertoires but typically fail to generate paired full-length gene products for direct expression cloning and do not enable linking this data to cell phenotype information. To overcome these limitations, we established a high-throughput platform for the quantitative and qualitative analysis of human TCR repertoires that provides insights into the clonal and functional composition of human CD4+ and CD8+ αß T cells at the molecular and cellular level. The strategy is a powerful tool to qualitatively assess differences between antigen receptors of phenotypically defined αß T cell subsets, e.g. in immune responses to cancer, vaccination, or infection, and in autoimmune diseases.