Testing the accuracy of methods for reconstructing ancestral states of continuous characters.

Many methods are available for estimating ancestral values of continuous characteristics, but little is known about how well these methods perform. Here we compare six methods: linear parsimony, squared-change parsimony, one-parameter maximum likelihood (Brownian motion), two-parameter maximum likelihood (Ornstein-Uhlenbeck process), and independent comparisons with and without branch-length information. We apply these methods to data from 20 morphospecies of Pleistocene planktic Foraminifera in order to estimate ancestral size and shape variables, and compare these estimates with measurements on fossils close to the phylogenetic position of 13 ancestors. No method produced accurate estimates for any variable: estimates were consistently less good as predictors of the observed values than were the averages of the observed values. The two-parameter maximum-likelihood model consistently produces the most accurate size estimates overall. Estimation of ancestral sizes is confounded by an evolutionary trend towards increasing size. Shape showed no trend but was still estimated very poorly: we consider possible reasons. We discuss the implications of our results for the use of estimates of ancestral characteristics.     

Persistence of infection in mice inoculated intranasally with Cryptococcus neoformans

Cryptococcus neoformans was instilled intranasally into mice which were periodically sacrificed to determine the course of infection. Cryptococci persisted within the nasal passages throughout the 90 day study. Extranasal dissemination began 14–28 days after instillation and was still demonstrable 90 days post-exposure. Ten percent mortality was observed in mice receiving 10⁶ cryptococci, while no mortality was observed in mice exposed to 10³ or 10⁴ cryptococci. Our research suggests that nasal colonization with C. neoformans can precede pulmonary and systemic cryptococcosis by weeks or months.     

Identification and characterization of a B cell activation factor (BCAF) produced by a human T cell line

A human T cell line, Peer, that expresses the T cell helper phenotype produces discrete activation and growth factors for tonsillar B cells. The B cell activation factor produced by Peer is biochemically and physiologically distinct from other lymphokines known to enhance B cell proliferation, namely, interleukin 1, interleukin 2, interferon, and previously characterized B cell growth factors (BCGF). The BCGF produced by Peer is functionally similar to previously described BCGF but has a m.w. of approximately 30,000 daltons. The identification and characterization of a T cell-derived activation factor that can induce apparently resting (Go phase) B cells to enter S phase in the absence of an exogenous first signal has important implications in the additional dissection of the complex steps in the human B cell cycle.     

Characterization of a T4+/Leu-8+ T cell clone that directly helps B cell Ig production by secreting B cell differentiation factor

A human T4+/Leu-8+ T cell clone (YA2) was established by phytohemagglutinin activation and interleukin 2 (IL 2) propagation. Functional characterization of this clone demonstrated that it provided potent help towards Ig production by pokeweed mitogen-stimulated B cells in the presence of small numbers of autologous T cells or by Staphylococcus aureus Cowan I (SAC)-activated B cells in the presence of B cell growth factor (BCGF). YA2 provided no help to resting B cells and minimal help to either unactivated B cells cultured with BCGF or SAC-activated B cells. Supernatant generated from clone YA2 by IL 2 stimulation had significant B cell differentiation activity but no BCGF or IL 2 activity. Thus, YA2 is a T4+/Leu-8+ potent direct helper only to B cells that are activated and proliferating due to its selective secretion of a differentiation factor, and not an activation and growth factor. The availability of phenotypically defined cloned populations of T cells with restricted functional helper activity related to the secretion of selected B cell tropic factors should prove useful in the dissection of the role of individual T cell subsets in the regulation of the human B cell cycle.     

Characterization of monoclonal B cell growth factor (BCGF) produced by a human T-T hybridoma

We previously demonstrated the development of a cloned human T cell hybridoma that secretes B cell growth factor (BCGF) in the absence of demonstrable interleukin 2 or B cell differentiation factor. Sephadex gel filtration chromatography demonstrated the m.w. of this factor to be 18 to 20K. The present studies were performed to further characterize the biochemical properties of the molecule and to determine its target cell specificity. Temperature stability studies showed the monoclonal BCGF to be stable at 37 degrees C for 12 hr and at 70 degrees C for 15 min; however, most (93%) of the activity was lost after incubation at 70 degrees C for 30 min. Aliquots of hybridoma supernatant were exposed to buffer solutions with variable pH with no diminution in activity over a pH range of 4.0 to 10.0 BCGF activity was not affected by 2-mercaptoethanol, neuraminidase, or nucleic acid denaturing enzymes. In contrast, all activity was destroyed by 10 M urea, trypsin, and chymotrypsin. Chromatofocusing demonstrated the isoelectric point of BCGF to be 6.3 to 6.6. Finally, absorption experiments demonstrated that BCGF activity was absorbed by large, activated B cells. Mitogen-stimulated T cell blasts, small resting B cells, and CESS cells failed to absorb BCGF activity from the hybridoma supernatant. These and future studies with purified monoclonal human BCGF should enhance our understanding of its immunochemical properties and of its role in the immunoregulation of human B cell responses.