Prolongation of allograft survival by 1,25-dihydroxyvitamin D3

BACKGROUND: 1,25-Dihydroxyvitamin D3, the hormonal form of vitamin D, is now believed to play a significant role in the immune responses, both in vitro and in vivo, preventing the development of several autoimmune diseases. These studies suggest that 1,25-dihydroxyvitamin D3 may be effective in prolonging allograph survival. METHODS: To test the hypothesis that 1,25-dihydroxyvitamin D3 would prolong allograft survival, neonatal heart grafts were transplanted to allogeneic recipients receiving either 19-nor-1,25-dihydroxyvitamin D2 (200 ng/day) or 1,25-dihydroxyvitamin D3 (50 ng/mouse/day) orally through the diet. The efficacy of 1,25-dihydroxyvitamin D3 in prolonging graft survival in a vascularized model was determined by heterotopic ACI to Lewis heart transplants. RESULTS: The provision of exogenous 1,25-dihydroxyvitamin D3 or an analog, 19-nor-1,25-dihydroxyvitamin D2, to mice markedly prolonged the survival of neonatal mouse heart allografts. Similar results were obtained with a vascularized heterotopic heart transplant model in rats. Cyclosporine at a maximum 25 mg/kg dose for mice proved less effective than 1,25-dihydroxyvitamin D3. Graft survival in mice differing at class I and class II loci (B10.A(4R) --> C57BL/10) increased from 13.0+/-1.1 days to 51.0+/-5.6 days and was significantly better than cyclosporine monotherapy (33.2+/-3.6). Rat heart survival in a high responder strain combination (ACI --> Lewis) increased from 6.2+/-0.3 to 25.2+/-2.8 days. The increased survival of the transplants brought about with 1,25-dihydroxyvitamin D3 was not accompanied by hypercalcemia in rats. CONCLUSION: These results suggest that 1,25-dihydroxyvitamin D3 can be used as an effective agent in preventing graft rejection.     

A Feedback Receive Amplifier for Optical Transmission Systems

We consider the design and implementation of a lownoise feedback receive amplifier for use in optical communication systems. The feedback amplifier configuration is advantageous in that it offers a wide bandwidth with good signal to noise ratio.     

A Modified Receiver for Optical Transmission Systems

A Percival coil connected between the photodetector and amplifier in the receiver of an optical communication system offers the possibility of improved signal-to-noise ratios (SNR's). When the photodetector is an avalanche diode, the modified receiver requires a lower value of avalanche gain for optimum performance.     

N-point processing of optical fibre backscatter signals

N-point processing, where many samples are taken from a single optical fibre backscatter return, is shown to be an efficient means of implementing a two-point processor. This method is unaffected by source noise peculiar to single heterostructure lasers, which is shown to limit severely the performance of boxcar integrators.     

Source-drive optimisation for optical-fibre systems using l.e.d.s

The influence of the source-drive duty factor on the performance of a digital fibre-optic communication system is explored. The system considered utilises an l.e.d. source and either a p?i?n or avalanche photodetector. It is shown that, for a fibre with known dispersion, an optimum duty factor exists which offers significant s.n.r. advantages over a system with impulsive excitation.     

Adaptive statistical encoding of d.p.c.m.-coded videotelephone signals

Adaptive statistical encoding of d.p.c.m.-coded video-telephone signals is considered. The need for an adaptive statistical encoder to operate on short-term batch statistics is explained and the effect on the bit rate of limiting the number of codes employed by the statistical encoder is reported.     

Intersymbol Interference with Orthogonal Signaling

The occurrence of both inter- and intrasymbol interference in a communications system utilizing orthogonal signals and memoryless correlation detection is described, and a system model is presented. A relationship between a bound on peak interference and time-bandwidth product for several well known orthogonal signal sets is reported.     

Receiver Design for Multilevel Digital Optical Fiber Systems

We consider the design of a receiver for a digital optical fiber system operating with multilevel pulse amplitude modulation. This form of modulation provides for high bit rates on digital systems employing large diameter optical fibers and LED sources. The receiver configuration analysis is one with a p-i-n avalanche photodetector and a FET preamplifier. The analysis is complicated by the fact that the signal levels and threshold boundries have to be nonuniformly spaced to cope with signal dependent shot noise introduced by the photodiode detector. An algorithm for level placement is reported. Performance figures for typical systems, with optical pulse shape which are either Gaussian or exponential, are calculated.     

Receiver Design for Optical PPM Systems

An optimum receiver structure for optical PPM systemics is considered and from this a simple sub-optimum receiver is proposed which offers significant SNR improvement relative to existing design approaches and yet requires few circuit changes. Receivers incorporating either p-i-n or avalanche photodiodes and FET or BJT preamplifiers are analyzed. The optimization technique relies on matching the frequency response of the receiver to that of the incident optical pulse and setting the avalanche gain and threshold detector at optimum levels. For a FET preamplifier the SNR improvement is 10.8 dB with a p-i-n photodiode and 4.7 dB with an avalanche photodiode. For a BJT preamplifier the SNR improvement is 3.1 dB and 2.3 dB with the p-i-n and avalanche diodes, respectively.     

Referred impedance noise analysis for feedback amplifiers

A simple approach for calculating the equivalent input noise spectral density of feedback amplifiers is proposed. The method is particularly relevant to the design of amplifiers for use in optical-communication receivers.