Fate of T and B cells transferred to SCID mice

Challenges and opportunities in targeting the CD28/CTLA-4 pathway in transplantation and autoimmunity

INTRODUCTION

T cell activation is a complex process that requires multiple cell signaling pathways, including a primary recognition signal and additional costimulatory signals. One of the best-characterized costimulatory pathways includes the Ig superfamily members CD28 and CTLA-4 and their ligands CD80 and CD86. Areas covered: This review discusses past, current and future biological therapies that have been utilized to block the CD28/CTLA-4 cosignaling pathway in the settings of autoimmunity and transplantation, as well the challenges facing successful implementation of these therapies. Expert opinion: The development of CD28 blockers Abatacept and Belatacept provided a more targeted therapy approach for transplant rejection and autoimmune disease relative to calcineurin inhibitors and anti-proliferatives, but overall efficacy may be limited due to their collateral effect of simultaneously blocking CTLA-4 coinhibitory signals. As such, current investigations into the potential of selective CD28 blockade to block the costimulatory potential of CD28 while exploiting the coinhibitory effects of CTLA-4 are promising. However, as selective CD28 blockade inhibits the activity of both effector and regulatory T cells, an important goal for the future is the design of therapies that will maximize the attenuation of effector responses while preserving the suppressive function of T regulatory cells.

Reconstitution of a Hyperinflammatory Prostaglandin E2Response toPorphyromonas gingivalisChallenge in Severe Combined Immunodeficient Mice

BACKGROUND

The purpose of this study was to determine whether lymphocytic cells regulate the monocytic hyperinflammatory trait (MO+) in chronic periodontitis patients. Using a P. gingivalis challenge model in severe combined immunodeficient (SCID) mice, we tested the effects of adoptively transferred human peripheral blood leukocytes from gingivitis and chronic periodontitis diabetic and non-diabetic individuals on monocytic responses.

METHODS

This response was examined using the subcutaneous tissue chamber infection model. Three weeks following cell reconstitution, all SCID mice were challenged with 10(9) colony forming units of live P. gingivalis HG405. Chamber contents were collected at day 7 after bacterial challenge for prostaglandin E2 (PGE2) analysis and chamber rejection monitored up to day 30. Gingival crevicular fluid (GCF) samples were collected from all patients for PGE2 analysis. Both chamber fluid- and GCF-PGE2 levels were determined by enzyme-linked immunosorbent assays (ELISA).

RESULTS

Significantly elevated GCF-PGE2 levels were found in diabetic as well as in non-diabetic patients with moderate/advanced periodontitis compared to diabetic and non-diabetic subjects with gingivitis/mild periodontitis at P= 0.01 and P= 0.001, respectively. As reflected in chamber fluid PGE2 levels and percentage chamber rejection, lymphocytic sensitization to P. gingivalis occurred in both diabetics and non-diabetics with moderate/advanced periodontitis, but not in diabetics and non-diabetics with gingivitis/mild periodontitis.

CONCLUSIONS

These results suggest that the exaggerated monocytic inflammatory response trait (MO+) associated with moderate/advanced chronic periodontitis is due, at least in part, to lymphocytic modulation, while the directional findings for lymphocytes from diabetic subjects deserve further investigation. Our findings further demonstrate that the SCID mouse model is a useful animal model to study human immune responses to periodontal microorganisms.

Naive B lymphocytes undergo homeostatic proliferation in response to B cell deficit

Naive peripheral B cells are maintained in sufficient numbers and diversity to mount effective immune responses against infectious agents. However, the size and repertoire of this B cell pool is constantly diminished by normal cell turnover and Ag activation. Homeostatic (Ag-independent) proliferation in response to B cell depletion is one mechanism to compensate for this cell loss. We have used purified CFSE-labeled B cells and an adoptive transfer model system to show that immature and mature B cells divide in a variety of B cell-deficient (scid, xid, IL-7(-/-), and sublethally irradiated) hosts. Homeostatic B cell proliferation is T cell independent, and B cells that have replicated by this mechanism retain the antigenic phenotype of naive B cells. Replication is significantly reduced in B cell-sufficient normal or B cell-reconstituted immunodeficient recipients by the action of competing mature follicular B cells. Using xid mice and transcription factor knockouts, we show that the activation signal(s) that lead to homeostatic B cell proliferation require Bruton's tyrosine kinase; however, c-Rel, a Bruton's tyrosine kinase-induced NF-kappaB/Rel transcription factor critical for Ag and mitogen stimulation, is dispensable, indicating the uniqueness of this activation pathway. Survival and replication signals can also be separated, because the transcription factor p50 (NF-kappaB1), which is required for the survival of peripheral B cells, is not necessary for homeostatic replication. Homeostatic B cell proliferation provides an Ag-independent mechanism for the maintenance and expansion of naive B cells selected into the mature B cell pool.

Self-limiting systemic autoimmune disease during reconstitution of T cell-deficient mice with syngeneic T cells: support for a multifaceted role of T cells in the maintenance of peripheral B cell tolerance

The T cell compartment can be partially reconstituted in mice with targeted inactivation of the TCR C(beta) and C(delta) genes by injection of mature, syngeneic T cells. Surprisingly, during this reconstitution high titers of IgG anti-nuclear antibodies and symptoms of systemic autoimmune disease develop. However, this autoimmune response is transient and aged, reconstituted mice show no overt signs of disease. The autoantibody response appears to be derived from a pre-existing population of host self-reactive B cells and requires CD40 ligand-mediated co-stimulation from donor cells. Diminution of this response is coincident with a vigorous germinal center reaction and the disappearance of a subpopulation of activated B cells that expresses elevated levels of Fas. Collectively, our data support the idea that T cells play a multifaceted role in the maintenance of peripheral B cell tolerance that includes mediating the activation-induced death of autospecific B cells.

Selecting and maintaining a diverse T-cell repertoire

To provide a T-cell population that will respond promptly to foreign antigen, the immune system looks inward, using the variety of self-antigens to select and maintain a diverse repertoire of receptors. A protective immune system must include a T-lymphocyte population that is poised to respond to foreign antigenic peptides presented by self-major histocompatibility complex molecules. As the organism cannot predict the precise pathogen-derived antigens that will be encountered, the system uses the diverse array of self-peptides bound to self-major histocompatibility complex molecules, not only to select a receptor repertoire in the thymus, but also to keep naïve T cells alive and 'ready for action' in the periphery.