Extrafollicular Plasmablasts Present in the Acute Phase of Infections Express High Levels of PD-L1 and Are Able to Limit T Cell Response.

During infections with protozoan parasites or some viruses, T cell immunosuppression is generated simultaneously with a high B cell activation. It has been described that, as well as producing antibodies, plasmablasts, the differentiation product of activated B cells, can condition the development of protective immunity in infections. Here, we show that, in T. cruzi infection, all the plasmablasts detected during the acute phase of the infection had higher surface expression of PD-L1 than other mononuclear cells. PD-L1hi plasmablasts were induced in vivo in a BCR-specific manner and required help from Bcl-6+CD4+T cells. PD-L1hi expression was not a characteristic of all antibody-secreting cells since plasma cells found during the chronic phase of infection expressed PD-L1 but at lower levels. PD-L1hi plasmablasts were also present in mice infected with Plasmodium or with lymphocytic choriomeningitis virus, but not in mice with autoimmune disorders or immunized with T cell-dependent antigens. In vitro experiments showed that PD-L1hi plasmablasts suppressed the T cell response, partially via PD-L1. Thus, this study reveals that extrafollicular PD-L1hi plasmablasts, whose peaks of response precede the peak of germinal center response, may have a modulatory function in infections, thus influencing T cell response.

A short hairpin RNA-based adjuvant targeting NF-κB repressor IκBα promotes migration of dermal dendritic cells to draining lymph nodes and antitumor CTL responses induced by DNA vaccination.

DNA vaccination is an attractive approach to elicit tumor-specific cytotoxic CD8+ T lymphocytes (CTL), which can mediate protective immunity against tumors. To initiate CTL responses, antigen-encoding plasmids employed for DNA vaccination need to activate dendritic cells (DC) through the stimulation of DNA-sensing innate immune receptors that converge in the activation of the master transcription factor NF-κB. To this end, NF-κB repressor IκBα needs to be degraded, allowing NF-κB to translocate to the nucleus and transcribe proinflammatory target genes, as well as its repressor IκBα. Therefore, NF-κB activation is self-limited by de novo synthesis of IκBa, which sequesters NF-κB in the cytosol. Hence, we tested whether co-delivering a shRNA-based adjuvant able to silence IκBα expression would further promote DNA-induced NFκB activation, DC activation and tumor-protective CTL responses induced by DNA vaccination in a preclinical model. First, an IκBα-targeting shRNA plasmid (shIκBα) was shown to reduce IκBα expression and promote NFκB-driven transcription in vitro, as well as up-regulate inflammatory target genes in vivo. Then, we showed that intradermal DNA electroporation induced the migration of skin migratory dendritic cells to draining lymph nodes and maturation of dermal dendritic cells (dDC). Interestingly, shIκBα further promoted the migration of mature skin migratory dendritic cells, in particular dDC, which are specialized in antigen cross-presentation and activation of CD8+ T cells. Consistently, mice vaccinated with a plasmid encoding the melanoma-associated antigen tyrosinase-related protein 2 (TRP2) in combination with shIκBα enhanced TRP2-specific CTL responses and reduced the number of lung melanoma foci in mice challenged with intravenous injection of B16F10 cells. Moreover, therapeutic vaccination with pTRP2 and shIκBα delayed the growth of B16F10 melanoma subcutaneous tumors. Our data suggest that adjuvants promoting NF-κB activation represent an attractive strategy to boost DC activation and promote the generation of tumor-protective CTL responses elicited by DNA vaccines.

Harnessing DNA-induced immune responses for improving cancer vaccines.

DNA vaccines have emerged as an attractive strategy to promote protective cellular and humoral immunity against the encoded antigen. DNA vaccines are easy to generate, inexpensive to produce and purify at large-scale, highly stable and safe. In addition, plasmids used for DNA vaccines act as powerful "danger signals" by stimulating several DNA-sensing innate immune receptors that promote the induction of protective adaptive immunity. The induction of tumor-specific immune responses represents a major challenge for DNA vaccines because most of tumor-associated antigens are normal non-mutated self-antigens. As a consequence, induction of potentially self-reactive T cell responses against such poorly immunogenic antigens is controlled by mechanisms of central and peripheral tolerance as well as tumor-induced immunosuppression. Although several DNA vaccines against cancer have reached clinical testing, disappointing results have been observed. Therefore, the development of new adjuvants that strongly stimulate the induction of antitumor T cell immunity and counteract immune-suppressive regulation is an attractive approach to enhance the potency of DNA vaccines and overcome tumor-associated tolerance. Understanding the DNA-sensing signaling pathways of innate immunity that mediate the induction of T cell responses elicited by DNA vaccines represents a unique opportunity to develop novel adjuvants that enhance vaccine potency. The advance of DNA adjuvants needs to be complemented with the development of potent delivery systems, in order to step toward successful clinical application. Here, we briefly discuss recent evidence showing how to harness DNA-induced immune response to improve the potency of cancer vaccines and counteract tumor-associated tolerance.