Membrane association and destabilization by Aggregatibacter actinomycetemcomitans leukotoxin requires changes in secondary structures

Aggregatibacter actinomycetemcomitans is a common inhabitant of the upper aerodigestive tract of humans and non-human primates and is associated with disseminated infections, including lung and brain abscesses, pediatric infective endocarditis, and localized aggressive periodontitis. Aggregatibacter actinomycetemcomitans secretes a repeats-in-toxin protein, leukotoxin, which exclusively kills lymphocyte function-associated antigen-1-bearing cells. The toxin's pathological mechanism is not fully understood; however, experimental evidence indicates that it involves the association with and subsequent destabilization of the target cell's plasma membrane. We have long hypothesized that leukotoxin secondary structure is strongly correlated with membrane association and destabilization. In this study, we tested this hypothesis by analysing lipid-induced changes in leukotoxin conformation. Upon incubation of leukotoxin with lipids that favor leukotoxin-membrane association, we observed an increase in leukotoxin α-helical content that was not observed with lipids that favor membrane destabilization. The change in leukotoxin conformation after incubation with these lipids suggests that membrane binding and membrane destabilization have distinct secondary structural requirements, suggesting that they are independent events. These studies provide insight into the mechanism of cell damage that leads to disease progression by A. actinomycetemcomitans.

Expression of major histocompatibility complex antigens in squamous cell carcinomas of the head and neck: effects of interferon gene transfer

The effect of retroviral-mediated interferon-gamma (IFN-gamma) gene transfer on major histocompatibility complex (MHC) class I and II antigen expression was investigated in 13 head and neck squamous carcinoma cell lines. Six cell lines exhibited increased MHC class I expression, and 10 exhibited increased MHC class II expression after IFN-gamma gene transfer. Differences in MHC antigen expression between parental and transduced cell lines were significant (P = 0. 002) only for cell lines that upregulated MHC class II expression. After incubation in medium containing 100 U/mL recombinant IFN-gamma, or in medium from IFN-gamma retrovirus-transduced NIH 3T3 cells, 12 cell lines significantly upregulated MHC class I expression, and 9 significantly upregulated MHC class II expression. Only cell lines that exhibited increased MHC class II expression after retroviral transduction also upregulated class II expression with exogenous IFN-gamma treatment. Thus some head and neck squamous carcinoma cell lines can upregulate MHC class I and II expression after exogenous application of either IFN-gamma or IFN-gamma retroviral transduction. These are promising findings for head and neck cancer immunotherapy and gene therapy.