Subtle Stimulation Succeeds
The synthetic glycolipid α-galactosylceramide (α-GalCer) is widely used as the model Ag to study the activity of type I NKT (iNKT) cells. This subpopulation of T cells is characterized by the coexpression of a semi-invariant TCR and NK cell markers and recognizes lipid Ags presented by CD1d molecules. Studies of the therapeutic potential of α-GalCer as an immunomodulatory agent have been hindered by the glycolipid’s powerful iNKT stimulatory properties. In their search for less potent structural analogs of α-GalCer, Kerzerho et al. (p. 2254) analyzed iNKT cell responses to HS44, a synthetic aminocyclitolic ceramide. HS44 administration in vivo selectively induced strong production of IFN-γ, which resulted in tumor inhibition in the murine B16F10 melanoma model. In addition, intranasal administration of HS44 caused airway inflammation, although it was reduced compared with the response observed in α-GalCer–sensitized mice. These data suggest that HS44 administration can effectively elicit both Th1 and Th2 responses. Kinetic and structural studies demonstrated that HS44 and α-GalCer had similar association rates for the TCR. However, HS44 had a much faster dissociation rate, most likely due to the formation of a reduced number of hydrogen bonds between the glycolipid and the TCR. The authors conclude that the lower TCR binding affinity of HS44 and its consequent weaker potency warrant future studies to explore its therapeutic use as an immunomodulator of iNKT cells.
In addition to the delivery of spermatozoa, seminal fluid has important immunological functions, such as the induction of immune tolerance toward the embryo. Studies in mice and other animals have also indicated that semen exposure contributes to the induction of an inflammatory response that positively affects the initiation of pregnancy. In this issue, Sharkey et al. (p. 2445) evaluated human immune responses to seminal fluid by examining cellular changes within the ectocervix of ovulating women before and after unprotected vaginal coitus, condom-protected coitus, or no coitus. Exposure to semen increased the numbers of activated CD14+ macrophages, CD1a+ dendritic cells, and CD3+CD8+CD45RO+ memory T cells. It also enhanced the cervical expression of the genes encoding GM-CSF, IL-1A, IL-6, and IL-8. Similarly, microarray data demonstrated elevated expression of a wide range of genes associated with inflammation and immunity, including those encoding chemokines and matrix metalloproteinases. These findings attest that human responses to seminal fluid are similar to those described in experimental animals. Moreover, they suggest that the mucosal microenvironment induced by unprotected coitus may influence resistance to local infections and/or cancer susceptibility.
Continuous Ag stimulation is believed to inactivate T cells during many chronic viral infections, ultimately contributing to pathogen persistence. In this issue, Mackay et al. (p. 2173) explored the functionality of effector T cells during persistent HSV infection. In contrast to other viruses, members of the herpesvirus family can persist in the infected host in a state of latency with little or no replication, and HSV generates a latent infection localized to the sensory ganglia that innervate the site of the initial infection. In their murine experimental system, the authors demonstrate that T cells in latently infected ganglia continued to express granzyme B, a molecule contributing to the inhibition of virus reactivation. The HSV-specific T cells from latently infected ganglia produced IFN-γ and TNF-α and expressed CD107 at levels similar to those in polyfunctional T cells recovered from ganglia during the acute phase of the infection. These HSV-specific T cells from latent infection expressed low levels of markers associated with cell exhaustion, including PD-1, and were able to proliferate after repeated cycles of virus reactivation. Overall, this study demonstrates that T cells associated with latent HSV infection retain full functional activity despite undergoing chronic antigenic stimulation.
Notch Signaling in the Gut
Intestinal epithelial cells (IECs) differentiate into absorptive and secretory lineages and are characterized by rapid proliferation, which is considered important to innate defense against infections. Balance in the cellular composition of the epithelial mucosal surface is essential for the maintenance of physiological conditions, and its disruption is associated with impaired host defense and gastrointestinal diseases. Notch signaling has been implicated in the molecular pathways regulating IEC function, either as a contributor to or inhibitor of colitis. To clarify the role of Notch in gut homeostasis, Obata et al. (p. 2427) generated the RBP-JΔIEC mouse strain whose epithelial cells lack expression of recombination signaling binding protein for the immunoglobulin κ J region (RBP-J), a transcription factor involved in Notch signaling. The mice developed spontaneous colitis with infiltration of Th17 cells, CD11b+ cells, and CD11c+ dendritic cells. Antibiotic treatment prevented colitis, indicating that intestinal bacteria were responsible for disease development. IECs of RBP-JΔIEC mice had impaired barrier function, as confirmed by detection of bacterial translocation in the murine colonic mucosa. In addition, loss of Notch signaling significantly retarded IEC turnover, leading to compromised epithelial defense mechanisms. Taken together, these results provide mechanistic insights into the involvement of Notch signaling in gastrointestinal immunity.
Revealing the Repertoire
The generation of diversity in the mouse Ab repertoire is far from simple and has been difficult to thoroughly address experimentally. In this issue, Aoki-Ota et al. (p. 2305) used deep sequencing methods to undertake a comprehensive evaluation of the Igκ repertoire in C57BL/6 mice. Analysis of the preimmune repertoire in bone marrow (BM), spleen, and lymph nodes revealed that 101 Vκ and 4 Jκ gene segments were used. Although ∼98.5% of the possible V–J combinations were observed, use of 7 Vκ genes comprised >40% of the repertoire and Jκ usage followed a stable hierarchy. Gene usage patterns in peripheral B cells were generally similar to those observed in BM B cells, although a subset of low-frequency BM rearrangements was not found in the periphery. V–J rearrangements resulting in both functional and nonfunctional transcripts were found to preferentially use the same Vκ and Jκ gene segments. The bias toward specific Vκ gene segment usage was not related to chromosomal position, proximity to the Jκ cluster, coding end sequences, or selection. Analysis of the repertoires of a series of genetic models demonstrated some alterations in gene segment usage when receptor editing was favored or secondary rearrangement patterns were altered. However, many Vκ segments consistently associated with the same Jκ segments and the overall gene usage patterns were remarkably stable. This study reveals significant skewing in the Vκ repertoire and will provide a framework for a much more complete understanding of Ab diversity than was previously possible.
Sweet on Memory B Cells
Multivalent polysaccharide vaccines against Streptococcus pneumoniae can offer some level of protection against pneumococcal disease in both children and adults, but individuals with deficiencies in IgM or memory B cell responses are poorly protected by vaccination. Khaskhely et al. (p. 2455) analyzed the phenotypes of pneumococcus polysaccharide (PPS)-specific memory B cells in vaccine-naïve healthy adult volunteers given a 23-valent pneumococcal polysaccharide vaccine. Consistent with previous studies, Abs specific to pneumococcal polysaccharides 14 (PPS14) and 23F (PPS23F), especially IgG, rose significantly six days after vaccination and correlated with an increase in functional Abs with opsophagocytic activity. Analysis of B cells with fluorescently labeled PPS14 or PPS23F molecules in combination with B cell markers showed that the majority of prevaccination B cells had a naive CD27−IgM+ phenotype. A small portion of B cells were also specifically labeled with PPS14 or PPS23F and exhibited a naive CD27− phenotype. Immunization induced a significant increase in PPS14- or PPS23F-specific CD27+IgM+ memory B cells but did not significantly change the frequency of CD27+IgM− switched memory B cells. These findings suggest that CD27+IgM+ memory B cells are a critical population that expands in response to pneumococcal polysaccharide vaccination.
Summaries written by Bernardetta Nardelli, Ph.D.