Phase-matched high-order harmonic generation in pre-ionized noble gases.

One of the main difficulties of efficiently generating high-order harmonics in long neutral-gas targets is to reach the phase-matching conditions. The issue is that the medium cannot be sufficiently ionized by the driving laser due to plasma defocusing. We propose a method to improve the phase-matching by pre-ionizing the gas using a weak capillary discharge. We have demonstrated this mechanism, for the first time, in absorption-limited XUV generation by an 800 nm femtosecond laser in argon and krypton. The ability to control phase-mismatch is confirmed by an analytical model and numerical simulations of the entire generation process. Our method allows to increase the efficiency of the harmonic generation significantly, paving the way towards photon-hungry applications of these compact short-wavelength sources.

[Experimental correction of irregular astigmatism in patients with keratoconus using diode laser thermal keratoplasty]. / Experimentální korekce nepravidelného astigmatismu u pacientu s keratokonem termokeratoplastikou diodovým laserem.

OBJECTIVE: To evaluate the potential of diode laser thermokeratoplasty (DTK) in the correction of irregular astigmatism in patients with keratoconus. GROUP AND METHODS: The coagulation of deep layers of cornea stroma was made on four eyes of four patients suffering from advanced keratoconus at the mean age of 31 years by means of the infrared contact cw laser Prolaser Rodenstock 1.9 DTK. The aim of the treatment was to increase the curvature of cornea in flat meridians and to compensate flattening of cornea in steep meridians by way of forming traction strips between the contracted tissue of individual coagulation points. The changes on the cornea topography and changes of visual functions were determined. RESULTS: The diode laser keratoplasty is capable to induce sector increase of cornea curvature and thereby improve symmetry pictures of irregular astigmatism in keratoconus. For a definitive inclusion of DLK into the spectrum of clinically applicable procedures, other therapeutic studies are needed as well as larger groups with relatively homogeneous pre-operation pictures.

Urocanic acid enhances IL-10 production in activated CD4+ T cells.

The immunosuppressive effects of UV radiation have been well documented. This suppression has been attributed to the action of the cis form of urocanic acid (UCA), a photoproduct of trans-UCA, a natural constituent of the skin. Here, we show that mouse spleen cells preincubated with cis-UCA have a diminished proliferative response to allogeneic cells in MLC and to stimulation with anti-CD3 mAb. Cells preincubated with cis-UCA also had a decreased ability to serve as APC and to stimulate the proliferation of allogeneic lymphocytes in MLC. Simultaneously, the production of IL-2 and IFN-gamma by cells preincubated with cis-UCA was decreased. However, IL-10 gene expression and IL-10 protein secretion by spleen cells stimulated in the presence of cis-UCA were significantly enhanced. The principal cell population displaying the cis-UCA-induced elevated production of IL-10 was CD4+ T cells, which were shown to be a direct target of cis-UCA action. This was also supported by the observation that production of IL-10 by stimulated splenic non-T cells or by macrophages was not altered by cis-UCA. The enhanced production of IL-10 by activated CD4+ T cells may represent a novel pathway of UVB radiation-induced, cis-UCA-mediated immunosuppression. We suggest that the elevated production of IL-10 by activated CD4+ T cells may account for the suppressor T cell phenomena described in UV-irradiated recipients.

The effect of urocanic acid on graft rejection in an experimental model of orthotopic corneal transplantation in rabbits.

BACKGROUND: Urocanic acid (UCA) is a natural component of the stratum corneum of the skin. It has been described as a photoreceptor for ultraviolet B radiation. UCA is present in the skin as a trans-isomer and undergoes UVB irradiation-dependent isomerization from trans-to cis-isomer. An immunosuppressive effect of irradiated UCA, i.e. a mixture of cis- and trans-isomers, has been demonstrated both in vivo and in vitro. The aim of this study was to evaluate an immunosuppressive effect of irradiated UCA on graft rejection in an experimental model of orthotopic corneal transplantation. METHOD: A commercially available UCA was dissolved in salt solution and irradiated by XeCl excimer laser beam in order to obtain a mixture of cis- and trans-isomers. The immunosuppressive effect of irradiated UCA, compared to controls, unirradiated UCA and salt solution, was evaluated in a high-risk orthotopic corneal transplantation model; the agents were administered subconjunctivally to rabbits. RESULTS: The rejection reaction was observed in all animals. The mean graft survival time in rabbits administered salt solution or unirradiated UCA was 20 days and 22 days, respectively. The irradiated solution of UCA significantly (P < 0.01, Mantel-Cox test) prolonged mean graft survival time to 29 days. CONCLUSION: Subconjunctival administration of irradiated UCA prolonged the graft survival time in comparison with unirradiated UCA or salt solution in recipients in a rabbit transplantation model. Although further studies are necessary, UCA seems to be an effective immunosuppressive drug after corneal transplantation.