Evaluation of immunoglobulins produced in vitro by head-kidney leucocytes of sea bass Dicentrarchus labrax by immunoenzymatic assay

A Cell-Based ELISA to Improve the Serological Analysis of Anti-SARS-CoV-2 IgG

Knowledge of the antibody-mediated immune response to SARS-CoV-2 is crucial to understand virus immunogenicity, establish seroprevalence, and determine whether subjects or recovered patients are at risk for infection/reinfection and would therefore benefit from vaccination. Here, we describe a novel and simple cell-ELISA specifically designed to measure viral spike S1-specific IgG produced in vitro by B cells in peripheral blood mononuclear cell (PBMC) cultures from a cohort of 45 asymptomatic (n = 24) and symptomatic (n = 21) individuals, with age ranging from 8 to 99 years. All subjects underwent ELISA serological screening twice, at the same time as the cell-ELISA (T2) as well as 35–60 days earlier (T1). Cryopreserved PBMCs of healthy donors obtained years before the COVID-19 pandemic were also included in the analysis. The preliminary results presented here show that out of 45 tested subjects, 16 individuals (35.5%) were positive to the cell-ELISA, 11 (24.5%) were concomitantly positive in the serological screening (T1 and/or T2), and only one person was exclusively positive in ELISA (T1) and negative in cell-ELISA, though values were close to the cutoff. Of note, five individuals (11.2%) tested negative in ELISA but positive in cell-ELISA and thus, they appear to have circulating B cells that produce antibodies against SARS-CoV-2, likely at levels that are undetectable in the serum, which challenges the negative results of the serological screening. The relative level of in vitro secreted IgG was measurable in positive subjects, ranging from 7 to 50 ng/well. Accordingly, all anti-SARS-CoV-2 antibody-positive subjects previously reported moderate to severe symptoms attributable to COVID-19, even though the RT-PCR data were rarely available to confirm viral infection. Overall, the described cell-ELISA might be an effective method for detecting subjects who encountered the virus in the past, and thus helpful to improve serological ELISA tests in the case of undetectable/equivocal circulating IgG levels, and a suitable and improved tool to better evaluate SARS-CoV-2-specific humoral immunity in the COVID-19 pandemic.

Fish Lymphocytes: An Evolutionary Equivalent of Mammalian Innate-Like Lymphocytes?

Lymphocytes are the responsible of adaptive responses, as they are classically described, but evidence shows that subpopulations of mammalian lymphocytes may behave as innate-like cells, engaging non-self rapidly and without antigen presentation. The innate-like lymphocytes of mammals have been mainly identified as γδT cells and B1-B cells, exert their activities principally in mucosal tissues, may be involved in human pathologies and their functions and tissue(s) of origin are not fully understood. Due to similarities in the morphology and immunobiology of immune system between fish and mammals, and to the uniqueness of having free-living larval stages where the development can be precisely monitored and engineered, teleost fish are proposed as an experimental model to investigate human immunity. However, the homology between fish lymphocytes and mammalian innate-like lymphocytes is an issue poorly considered in comparative immunology. Increasing experimental evidence suggests that fish lymphocytes could have developmental, morphological, and functional features in common with innate-like lymphocytes of mammals. Despite such similarities, information on possible links between conventional fish lymphocytes and mammalian innate-like lymphocytes is missing. The aim of this review is to summarize and describe available findings about the similarities between fish lymphocytes and mammalian innate-like lymphocytes, supporting the hypothesis that mammalian γδT cells and B1-B cells could be evolutionarily related to fish lymphocytes.

Ontogeny, tissue distribution and expression analysis of IgZ in rohu, Labeo rohita in response to various stimuli

This study reports presence of IgZ transcripts, its ontogeny, tissue distribution and expression following various stimulations/infections in rohu. The derived rohu IgZ sequence clustered together with IgZ of Ctenopharyngodon idella and Cyprinus carpio in phylogenetic analysis. IgZ expression was detected at early developmental stages with higher expression at 1 day post-fertilization, and higher in anterior kidney and lower in skin tissues of juveniles. An inductive expression of IgZ was observed during both Edwardsiella tarda and Dactylogyrus catlaius infections in skin and/or gill tissues. In ConA treated HKLs, the response was prominent at 72 h post-stimulation where as in ConA-PMA treatment, it was higher during early time points. Increased expression of IgZ was found between 24 and 96 hps with formalin-killed Aeromonas hydrophila where as in poly I:C treated HKLs, the level increased at 96 hps. It seems to be the first report describing the functional existence of IgZ in rohu carp.

Plasmablasts and plasma cells: reconsidering teleost immune system organization

Comparative immunologists have expended extensive efforts in the characterization of early fish B cell development; however, analysis of the post-antigen induction stages of antibody secreting cell (ASC) differentiation has been limited. In contrast, work with murine ASCs has resolved the physically and functionally distinct cells known as plasmablasts, the short-lived plasma cells and long-lived plasma cells. Teleost ASCs are now known to also possess comparable subpopulations, which can greatly differ in such basic functions as lifespan, antigen sensitivity, antibody secretion rate, differentiative potential, and distribution within the body. Understanding the mechanisms by which these subpopulations are produced and distributed is essential for both basic understanding in comparative immunology and practical vaccine engineering.

Cell markers and determinants in fish immunology

Despite the impressive increase in the cloning and expression of genes encoding fish immunoregulatory molecules, the knowledge on "in vivo" and "in vitro" functional immunology of the corresponding peptide products is still at an initial stage. This is partly due to the lacking of specific markers for immunoregulatory peptides, that represent an indispensible tool to dissect immune reactions and to trace the fate of cellular events downstream of the activation. In this review we summarise the available information on functional immune activities of some teleost species and discuss the obtained data in an evolutionary and applied context.

Plasmablast and Plasma Cell Production and Distribution in Trout Immune Tissues

These studies describe the in vitro and ex vivo generation of plasmablasts and plasma cells in trout (Oncorhynchus mykiss) peripheral blood and splenic and anterior kidney tissues. Cells were derived either from naive trout and cultured with the polyclonal activator, Escherichia coli LPS, or from trout that had been immunized with trinitrophenyl-keyhole limpet hemocyanin. Hydroxyurea was used to resolve populations of replicating (plasmablast) and nonreplicating (plasma cell) Ab-secreting cells (ASC). Complete inhibition of Ig secretion was only observed within the PBL. Both anterior kidney and splenic lymphocytes possessed a subset of ASCs that were hydroxyurea resistant. Thus, in vitro production of plasma cells appears to be restricted to the latter two tissues, whereas peripheral blood is exclusively restricted to the production of plasmablasts. After immunization with trinitrophenyl-keyhole limpet hemocyanin, specific ASC could be isolated from all immune organs; however, the anterior kidney contained 98% of all ASC. Late in the response (>10 wk), anterior kidney ASC secreted specific Ab for at least 15 days in culture, indicating that they were long-lived plasma cells. Cells from spleen and peripheral blood lost all capacity to secrete specific Ab in the absence of Ag. Late in the Ab response, high serum titer levels are solely the result of Ig secretion from anterior kidney plasma cells.