Determinants of cytoplasmic microtubule depolymerization during ciliogenesis in Chlamydomonas

At the core of cilia are microtubules which establish length and assist ciliary assembly and disassembly; however, microtubules outside of the cilium can regulate ciliogenesis. The microtubule cytoskeleton polymerizes and depolymerizes rapidly. These processes have been studied across various organisms with chemical and genetic perturbations. However, these have generated conflicting data in terms of the role of cytoplasmic microtubules (CytoMTs) and free tubulin dynamics during ciliogenesis. Here, we look at the relationship between ciliogenesis and CytoMT dynamics in Chlamydomonas reinhardtii using chemical and mechanical perturbations. We find that not only can stabilized CytoMTs allow for normal ciliary assembly, but high calcium concentrations and low pH-induced deciliation cause CytoMTs to depolymerize separately from ciliary shedding. In addition, ciliary shedding through mechanical shearing allows cilia to regenerate earlier despite intact CytoMTs. Our data suggest that CytoMTs are not a sink for a limiting pool of cytoplasmic tubulin in Chlamydomonas, depolymerization after deciliation is a consequence rather than a requirement for ciliogenesis, and intact tubulin in the cytoplasm and proximal cilium support more efficient ciliary assembly.

Determinants of cytoplasmic microtubule depolymerization during ciliogenesis in Chlamydomonas reinhardtii

At the core of cilia are microtubules which are important for establishing length and assisting ciliary assembly and disassembly; however, another role for microtubule regulation on ciliogenesis lies outside of the cilium. The microtubule cytoskeleton is a highly dynamic structure which polymerizes and depolymerizes rapidly to assist in cellular processes. These processes have been studied across various organisms with chemical as well as genetic perturbations. However, these have generated conflicting data in terms of the role of cytoplasmic microtubules (CytoMTs) and free tubulin dynamics during ciliogenesis. Here we look at the relationship between ciliogenesis and cytoplasmic microtubule dynamics in Chlamydomonas reinhardtii using chemical and mechanical perturbations. We find that not only can stabilized CytoMTs allow for normal ciliary assembly, but high calcium concentrations and low pH-induced deciliation cause CytoMTs to depolymerize separately from ciliary shedding. In addition, we find that ciliary shedding through mechanical shearing, cilia regenerate earlier despite intact CytoMTs. Our data suggests that cytoplasmic microtubules are not a sink for a limiting pool of cytoplasmic tubulin in Chlamydomonas, depolymerization that occurs following deciliation is a consequence rather than a requirement for ciliogenesis, and intact CytoMTs in the cytoplasm and the proximal cilium support more efficient ciliary assembly.

The ERK activator, BCI, inhibits ciliogenesis and causes defects in motor behavior, ciliary gating, and cytoskeletal rearrangement

MAPK pathways are well-known regulators of the cell cycle, but they have also been found to control ciliary length in a wide variety of organisms and cell types from Caenorhabditis elegans neurons to mammalian photoreceptors through unknown mechanisms. ERK1/2 is a MAP kinase in human cells that is predominantly phosphorylated by MEK1/2 and dephosphorylated by the phosphatase DUSP6. We have found that the ERK1/2 activator/DUSP6 inhibitor, (E)-2-benzylidene-3-(cyclohexylamino)-2,3-dihydro-1H-inden-1-one (BCI), inhibits ciliary maintenance in Chlamydomonas and hTERT-RPE1 cells and assembly in Chlamydomonas These effects involve inhibition of total protein synthesis, microtubule organization, membrane trafficking, and KAP-GFP motor dynamics. Our data provide evidence for various avenues for BCI-induced ciliary shortening and impaired ciliogenesis that gives mechanistic insight into how MAP kinases can regulate ciliary length.

Lithium-induced ciliary lengthening sparks Arp2/3 complex-dependent endocytosis

Ciliary length is highly regulated, but can be disrupted by lithium, which causes ciliary elongation across cell types and organisms. We used the algal system Chlamydomonas reinhardtii to investigate the mechanism behind lithium-induced ciliary elongation. Protein synthesis is not required for lengthening, and the target of lithium, GSK3, has substrates that can influence membrane dynamics. Further, ciliary assembly requires a supply of ciliary membrane as well as protein. Lithium-treated cilia elongate normally with brefeldin treatment, but dynasore treatment produced defective lengthening suggesting a source of membrane from the cell surface rather than the Golgi. Genetic or chemical perturbation of the Arp2/3 complex or dynamin, required for endocytosis, blocks lithium-induced ciliary lengthening. Finally, we found an increase in Arp2/3 complex- and endocytosis-dependent actin filaments near the ciliary base upon lithium treatment. Our results identify a mechanism for lithium-mediated cilium lengthening and demonstrate the endocytic pathway for cilium membrane supply in algae is likely a conserved mechanism given lithium's conserved effects across organisms.

Initial ciliary assembly in Chlamydomonas requires Arp2/3 complex-dependent endocytosis

Ciliary assembly, trafficking, and regulation are dependent on microtubules, but the mechanisms of ciliary assembly also require the actin cytoskeleton. Here, we dissect subcellular roles of actin in ciliogenesis by focusing on actin networks nucleated by the Arp2/3 complex in the powerful ciliary model, Chlamydomonas. We find that the Arp2/3 complex is required for the initial stages of ciliary assembly when protein and membrane are in high demand but cannot yet be supplied from the Golgi complex. We provide evidence for Arp2/3 complex-dependent endocytosis of ciliary proteins, an increase in endocytic activity upon induction of ciliary growth, and relocalization of plasma membrane proteins to newly formed cilia.

Comparative Phenotyping of Two Commonly Used Chlamydomonas reinhardtii Background Strains: CC-1690 (21gr) and CC-5325 (The CLiP Mutant Library Background)

The unicellular green alga Chlamydomonas reinhardtii is an excellent model organism to investigate many essential cellular processes in photosynthetic eukaryotes. Two commonly used background strains of Chlamydomonas are CC-1690 and CC-5325. CC-1690, also called 21gr, has been used for the Chlamydomonas genome project and several transcriptome analyses. CC-5325 is the background strain for the Chlamydomonas Library Project (CLiP). Photosynthetic performance in CC-5325 has not been evaluated in comparison with CC-1690. Additionally, CC-5325 is often considered to be cell-wall deficient, although detailed analysis is missing. The circadian rhythms in CC-5325 are also unclear. To fill these knowledge gaps and facilitate the use of the CLiP mutant library for various screens, we performed phenotypic comparisons between CC-1690 and CC-5325. Our results showed that CC-5325 grew faster heterotrophically in dark and equally well in mixotrophic liquid medium as compared to CC-1690. CC-5325 had lower photosynthetic efficiency and was more heat-sensitive than CC-1690. Furthermore, CC-5325 had an intact cell wall which had comparable integrity to that in CC-1690 but appeared to have reduced thickness. Additionally, CC-5325 could perform phototaxis, but could not maintain a sustained circadian rhythm of phototaxis as CC1690 did. Finally, in comparison to CC-1690, CC-5325 had longer cilia in the medium with acetate but slower swimming speed in the medium without nitrogen and acetate. Our results will be useful for researchers in the Chlamydomonas community to choose suitable background strains for mutant analysis and employ the CLiP mutant library for genome-wide mutant screens under appropriate conditions, especially in the areas of photosynthesis, thermotolerance, cell wall, and circadian rhythms.

A guide to setting up and managing a lab at a research-intensive institution

Postdocs who land faculty jobs at research-intensive institutions need to juggle several new large-scale tasks: identifying space and equipment needs for their lab, negotiating the hiring package, outfitting the lab with supplies, building a team, and learning to manage time in ways that can promote productivity and happiness. Here we share tips to help new hires think clearly about each of these tasks.

Separable roles for RanGTP in nuclear and ciliary trafficking of a kinesin-2 subunit

Kinesin is part of the microtubule-binding motor protein superfamily, which serves important roles in cell division and intraorganellar transport. The heterotrimeric kinesin-2, consisting of the heterodimeric motor subunits, kinesin family member 3A/3B (KIF3A/3B), and kinesin-associated protein 3 (KAP3), is highly conserved across species from the unicellular eukaryote Chlamydomonas to humans. It plays diverse roles in cargo transport including anterograde (base to tip) trafficking in cilia. However, the molecular determinants mediating trafficking of heterotrimeric kinesin-2 itself are poorly understood. It has been previously suggested that ciliary transport is analogous to nuclear transport mechanisms. Using Chlamydomonas and human telomerase reverse transcriptase-retinal pigment epithelial cell line, we show that RanGTP, a small GTPase that dictates nuclear transport, regulates ciliary trafficking of KAP3, a key component for functional kinesin-2. We found that the armadillo-repeat region 6 to 9 (ARM6-9) of KAP3, required for its nuclear translocation, is also necessary and sufficient for its targeting to the ciliary base. Given that KAP3 is essential for cilium formation and there are the emerging roles for RanGTP/importin β in ciliary protein targeting, we further investigated the effect of RanGTP in cilium formation and maintenance. We found that precise control of RanGTP levels, revealed by different Ran mutants, is crucial for cilium formation and maintenance. Most importantly, we were able to provide orthogonal support in an algal model system that segregates RanGTP regulation of ciliary protein trafficking from its nuclear roles. Our work provides important support for the model that nuclear import mechanisms have been co-opted for independent roles in ciliary import.

Finding your unique path in science

I always found it curious that in science, we value unique, creative thinkers, but we teach scientists to progress in a formulaic manner that rarely takes each person's individual strengths into account. Surprisingly, when we break the mold, we are often rewarded for it. This cycle of learning to survive using conventional wisdom but being rewarded for a unique path outside of it seems to be an unspoken key to success. I am honored to be awarded the 2020 Women in Cell Biology Junior Award for Excellence in Research and am thrilled to share some of the unconventional guiding principles that brought me to where I am in this rich scientific landscape. The game changers in the early phase of my career were informal mentors, open scientific communication, and persistence in pursuing difficult scientific questions.

Chlamydomonas reinhardtii formin FOR1 and profilin PRF1 are optimized for acute rapid actin filament assembly

The regulated assembly of multiple filamentous actin (F-actin) networks from an actin monomer pool is important for a variety of cellular processes. Chlamydomonas reinhardtii is a unicellular green alga expressing a conventional and divergent actin that is an emerging system for investigating the complex regulation of actin polymerization. One actin network that contains exclusively conventional F-actin in Chlamydomonas is the fertilization tubule, a mating structure at the apical cell surface in gametes. In addition to two actin genes, Chlamydomonas expresses a profilin (PRF1) and four formin genes (FOR1-4), one of which (FOR1) we have characterized for the first time. We found that unlike typical profilins, PRF1 prevents unwanted actin assembly by strongly inhibiting both F-actin nucleation and barbed-end elongation at equimolar concentrations to actin. However, FOR1 stimulates the assembly of rapidly elongating actin filaments from PRF1-bound actin. Furthermore, for1 and prf1-1 mutants, as well as the small molecule formin inhibitor SMIFH2, prevent fertilization tubule formation in gametes, suggesting that polymerization of F-actin for fertilization tubule formation is a primary function of FOR1. Together, these findings indicate that FOR1 and PRF1 cooperate to selectively and rapidly assemble F-actin at the right time and place.

The elusive actin cytoskeleton of a green alga expressing both conventional and divergent actins

The green alga Chlamydomonas reinhardtii is a leading model system to study photosynthesis, cilia, and the generation of biological products. The cytoskeleton plays important roles in all of these cellular processes, but to date, the filamentous actin network within Chlamydomonas has remained elusive. By optimizing labeling conditions, we can now visualize distinct linear actin filaments at the posterior of the nucleus in both live and fixed vegetative cells. Using in situ cryo-electron tomography, we confirmed this localization by directly imaging actin filaments within the native cellular environment. The fluorescently labeled structures are sensitive to the depolymerizing agent latrunculin B (Lat B), demonstrating the specificity of our optimized labeling method. Interestingly, Lat B treatment resulted in the formation of a transient ring-like filamentous actin structure around the nucleus. The assembly of this perinuclear ring is dependent upon a second actin isoform, NAP1, which is strongly up-regulated upon Lat B treatment and is insensitive to Lat B-induced depolymerization. Our study combines orthogonal strategies to provide the first detailed visual characterization of filamentous actins in Chlamydomonas, allowing insights into the coordinated functions of two actin isoforms expressed within the same cell.

Partially Redundant Actin Genes in Chlamydomonas Control Transition Zone Organization and Flagellum-Directed Traffic

The unicellular green alga Chlamydomonas reinhardtii is a biflagellated cell with two actin genes: one encoding a conventional actin (IDA5) and the other encoding a divergent novel actin-like protein (NAP1). Here, we probe how actin redundancy contributes to flagellar assembly. Disrupting a single actin allows complete flagellar assembly. However, when disrupting both actins using latrunculin B (LatB) treatment on the nap1 mutant background, we find that actins are necessary for flagellar growth from newly synthesized limiting flagellar proteins. Under total actin disruption, transmission electron microscopy identified an accumulation of Golgi-adjacent vesicles. We also find that there is a mislocalization of a key transition zone gating and ciliopathy protein, NPHP-4. Our experiments demonstrate that each stage of flagellar biogenesis requires redundant actin function to varying degrees, with an absolute requirement for these actins in transport of Golgi-adjacent vesicles and flagellar incorporation of newly synthesized proteins.

Visualizing Filamentous Actin Using Phalloidin in Chlamydomonas reinhardtii

This protocol aims to visualize the filamentous actin network in Chlamydomonas reinhardtii. We improved fixed-cell labeling conditions using the F-actin probe, phalloidin. We created a Chlamydomonas-optimized protocol by halving the phalloidin incubation time, electing for optimal fixation conditions, and selecting for a healthy cell population. This phalloidin protocol is quick, effective, and is the only labeling method to date that allows for reliable actin filament detection in fixed vegetative Chlamydomonas cells. This method reveals previously unidentified actin structures in Chlamydomonas and novel insights into cytoskeletal dynamics.

Can microtubule motors use every available track?

Prachee Avasthi discusses new work showing that only a subset of microtubule doublets are used for intraflagellar transport in trypanosomes.

Flagellar assembly and function depend on cargo traveling via motors on microtubule doublets. Bertiaux, Mallet et al. (2018. J. Cell Biol. https://doi.org/10.1083/jcb.201805030) find that only a subset of available doublets are used for this transport in trypanosomes, leading to questions about how and why this is achieved.

Journal clubs in the time of preprints

Early-career researchers can learn about peer review by discussing preprints at journal clubs and sending feedback to the authors.

Erratum to: Chemical Screening for Flagella-Associated Phenotypes in Chlamydomonas reinhardtii

The original version of this chapter was inadvertently published with the incorrect affiliation for Brittany Jack and Dr Prachee Avasthi. This has been corrected and the affiliation details has been updated as follows.

Ciliary regulation: disassembly takes the spotlight

The mechanisms that control organelle size are poorly understood. Genetic analysis now shows that a NIMA-related kinase helps to regulate the size of cilia by activating disassembly of the cilium when it gets too long.

Actin is required for IFT regulation in Chlamydomonas reinhardtii

Assembly of cilia and flagella requires intraflagellar transport (IFT), a highly regulated kinesin-based transport system that moves cargo from the basal body to the tip of flagella [1]. The recruitment of IFT components to basal bodies is a function of flagellar length, with increased recruitment in rapidly growing short flagella [2]. The molecular pathways regulating IFT are largely a mystery. Because actin network disruption leads to changes in ciliary length and number, actin has been proposed to have a role in ciliary assembly. However, the mechanisms involved are unknown. In Chlamydomonas reinhardtii, conventional actin is found in both the cell body and the inner dynein arm complexes within flagella [3, 4]. Previous work showed that treating Chlamydomonas cells with the actin-depolymerizing compound cytochalasin D resulted in reversible flagellar shortening [5], but how actin is related to flagellar length or assembly remains unknown. Here we utilize small-molecule inhibitors and genetic mutants to analyze the role of actin dynamics in flagellar assembly in Chlamydomonas reinhardtii. We demonstrate that actin plays a role in IFT recruitment to basal bodies during flagellar elongation and that when actin is perturbed, the normal dependence of IFT recruitment on flagellar length is lost. We also find that actin is required for sufficient entry of IFT material into flagella during assembly. These same effects are recapitulated with a myosin inhibitor, suggesting that actin may act via myosin in a pathway by which flagellar assembly is regulated by flagellar length.

Ciliary secretion: switching the cellular antenna to 'transmit'

Cilia are microtubule/membrane-based protrusions that mediate cell motility or transduce sensory information. New work in Chlamydomonas demonstrates that cilia can also act as secretory organelles by budding enzyme-containing vesicles from the flagellar membrane for post-mitotic hatching of daughters from the mother cell wall.

Germline deletion of Cetn1 causes infertility in male mice

Centrins are calmodulin-like Ca(2+)-binding proteins that can be found in all ciliated eukaryotic cells from yeast to mammals. Expressed in male germ cells and photoreceptors, centrin 1 (CETN1) resides in the photoreceptor transition zone and connecting cilium. To identify its function in mammals, we deleted Cetn1 by homologous recombination. Cetn1(-/-) mice were viable and showed no sign of retina degeneration suggesting that CETN1 is nonessential for photoreceptor ciliogenesis or structural maintenance. Phototransduction components localized normally to the Cetn1(-/-) photoreceptor outer segments, and loss of CETN1 had no effect on light-induced translocation of transducin to the inner segment. Although Cetn1(-/-) females and Cetn1(+/-) males had normal fertility, Cetn1(-/-) males were infertile. The Cetn1(-/-) testes size was normal, and spermatogonia as well as spermatocytes developed normally. However, spermatids lacked tails suggesting severe defects at the late maturation phase of spermiogenesis. Viable sperm cells were absent and the few surviving spermatozoa were malformed. Light and electron microscopy analyses of Cetn1(-/-) spermatids revealed failures in centriole rearrangement during basal body maturation and in the basal-body-nucleus connection. These results confirm an essential role for CETN1 in late steps of spermiogenesis and spermatid maturation.

A chemical screen identifies class a g-protein coupled receptors as regulators of cilia

Normal cilia length and motility are critical for proper cellular function. Prior studies of the regulation of ciliary structure and length have primarily focused on the intraflagellar transport machinery and motor proteins required for ciliary assembly and disassembly. However, several mutants with abnormal length flagella highlight the importance of signaling proteins as well. In this study, an unbiased chemical screen was performed to uncover signaling pathways that are critical for ciliogenesis and length regulation using flagella of the green alga Chlamydomonas reinhardtii as a model. The annotated Sigma LOPAC1280 chemical library was screened for effects on flagellar length, motility, and severing as well as cell viability. Assay data were clustered to identify pathways regulating flagella. The most frequent target found to be involved in flagellar length regulation was the family of dopamine binding G-protein coupled receptors (GPCRs). In mammalian cells, cilium length could indeed be altered with expression of the dopamine D1 receptor. Our screen thus reveals signaling pathways that are potentially critical for ciliary formation, resorption, and length maintenance, which represent candidate targets for therapeutic intervention of disorders involving ciliary malformation and malfunction.

Stages of ciliogenesis and regulation of ciliary length

Cilia and flagella are highly conserved eukaryotic microtubule-based organelles that protrude from the surface of most mammalian cells. These structures require large protein complexes and motors for distal addition of tubulin and extension of the ciliary membrane. In order for ciliogenesis to occur, coordination of many processes must take place. An intricate concert of cell cycle regulation, vesicular trafficking, and ciliary extension must all play out with accurate timing to produce a cilium. Here, we review the stages of ciliogenesis as well as regulation of the length of the assembled cilium. Regulation of ciliogenesis during cell cycle progression centers on centrioles, from which cilia extend upon maturation into basal bodies. Centriole maturation involves a shift from roles in cell division to cilium nucleation via migration to the cell surface and docking at the plasma membrane. Docking is dependent on a variety of proteinaceous structures, termed distal appendages, acquired by the mother centriole. Ciliary elongation by the process of intraflagellar transport (IFT) ensues. Direct modification of ciliary structures, as well as modulation of signal transduction pathways, play a role in maintenance of the cilium. All of these stages are tightly regulated to produce a cilium of the right size at the right time. Finally, we discuss the implications of abnormal ciliogenesis and ciliary length control in human disease as well as some open questions.

Cardiopulmonary responses to acute hypoxia, head-down tilt and fluid loading in anesthetized dogs

The separate and combined acute effects of hypoxia (HY-11% O2), head-down tilt (HD-30 degrees) and fluid loading (FL-1.0 L saline) on hemodynamics and pulmonary gas exchange were determined in 17 anesthetized, mechanically ventilated dogs. Both during HY and normoxia (NO), the total respiratory compliance was decreased by HD, attributable to pulmonary vascular congestion. The reductions in compliance were twice as great with FL, indicating pulmonary interstitial edema, which was supported by histological observation of lung tissue. Pressure-flow relationships in the pulmonary circulation indicated that superimposing HD on HY doubled the increase in vascular resistance due to HY alone, while in the systemic circulation the resistance was returned to below NO by HD. A significant positive correlation between the changes in blood volume and pulmonary artery pressure for experimental transitions suggests that a shift in blood volume from systemic to pulmonary circulations and changes in total blood volume probably contributed substantially to these apparent changes in resistance. Pulmonary gas exchange efficiency, whether expressed in terms of shunt or ventilation/perfusion distribution from arterial-end-tidal PCO2 and PO2 differences, showed a significant inverse relationship with pulmonary driving pressure for the experimental conditions imposed. No clear synergistic effects of HY on HD were evident in contributing to pulmonary edema when superimposed prior to FL, but after FL this risk must be considered.

Arteriovenous and capillary hemangiomas of the interventricular septum

The case of a patient with two extremely unusual types of hemangioma is presented. The arteriovenous and capillary hemangiomas of the interventricular septum were discovered on coronary angiography performed for chest pain. Echocardiography confirmed the diagnosis. Successful surgical excision was performed.

Efficacy of plasma exchange in severe idiopathic rapidly progressive glomerulonephritis. A report of ten cases

The vast majority of patients with idiopathic rapidly progressive glomerulonephritis (RPGN) develop irreversible renal failure within weeks to months. We report a retrospective study of 10 patients with idiopathic RPGN who were treated with plasma exchange in addition to steroids and immunosuppression. Renal biopsies were obtained in nine patients. RPGN without immune complexes was present in four, immune complex disease in four, and anti-glomerular basement membrane disease in two. Renal function did not recover in those patients with anti-glomerular basement membrane disease. In contrast, seven of eight patients without auto-antibodies to basement membranes responded to therapy, and four did not need dialysis for two years or more. The sustained improvement in renal function in these seven patients suggests the need for evaluation of plasma exchange as an adjunct to steroids and immunosuppression in a prospective, controlled study in RPGN.

Bladder urothelium in nephropathy associated with analgesics

A history of ingestion of large quantities of analgesics in patients having clinical and radiographic features of chronic interstitial nephritis leads to the diagnosis of analgesic nephropathy. In analgesic nephropathy the renal damage possibly occurs from a high concentration of analgesics and/or their metabolites. Since urinary concentrations of analgesics and their metabolites are also high, urinary bladder wall may be similarly damaged. Bladder urothelium was examined in 20 patients. Ten control patients with uremia, hypertension, or bladder neck obstruction showed normal histology. Two of the 5 patients with analgesic nephropathy had brownish pigmentation of bladder. All 5 analgesic nephropathy patients showed chronic inflammation similar to that found in 5 patients with bacterial infections. It is suggested that pigmentation of bladder wall and/or chronic inflammation of bladder mucosa of unknown etiology when found in a patient with chronic interstitial nephritis, would support the diagnosis of analgesic nephropathy.

Effect of cataract on intraocular pressure

Cases of immature and mature cataract were selected to study normal values in relation to hypermature cataract. We observed that cases of hypermature cataract had a high intraocular tension which was associated with a high protein level and increase in the number of macrophages in the aqueous. We concluded that a phacolytic mechanism is responsible for a secondary rise of intraocular tension in cases of hypermature cataract.

Glomerulonephritis in agammaglobulinaemia

Images

Experimental glomerulonephritis in the mouse. II. Effect of antithymocyte globulin

Experimental glomerulonephritis induced by heterologous GBM immunization in mice is temporarily prevented and reduced in severity when ATG is administered prior to or in conjunction with GBM. Once the disease is well established, ATG does not appear to significantly alter glomerular damage.

Effects of antilymphocyte serum in neonatally thymectomized mice

This report concerns the influence of the thymus on the response to antilymphocyte serum (ALS) in newborn mice of the Balb/C strain. Thymectomy or sham surgery was performed within 24 hours of birth and the 120 involved mice divided into the following experimental groups on the basis of the material subsequently injected: isotonic saline, normal rabbit serum, antilymphocyte serum, neonatal thymectomy, neonatal thymectomy plus normal rabbit serum and neonatal thymectomy plus antilymphocyte serum. Parameters evaluated included spleen index, mortality, absolute lymphocyte count, renal lysozyme activity, primary and secondary hemolysin titers. The results suggest that, in contradistinction to adult mice, the consequences of ALS administration to newborns are but slowly reversible and complete recovery may not be possible in the absence of the thymus.